As strange as the creatures of the microcosmos are, their lives still revolve around the same fundamentals that ours do.

There’s food, there's reproduction, and there's death. Yes, even microbes, hardy as they can be, experience death.

In some ways, you could say they invented it. And on our journey through the microcosmos, we’ve watched those deaths through many lenses. Some are slow, calm affairs, while others are explosive or creepy.

And today, we’re going to try something new for our channel. We have gathered a few of our favorite episodes about death in the microcosmos so that we can see where our journey has taken us. So yeah, this is the kind of video you can turn on, and leave on for awhile.

This first video is also one of our oldest, so you’ll notice that a lot of footage in it looks very different from what we show these days because thanks to the support of our viewers, we’ve been able to upgrade our microscope multiple times over the course of this show. So the microscope may be different. But the death, well, the death remains the same.

This round little unicellular creature came to us via a plankton net, a mesh with tiny, microscopic holes through which we ran hundreds and hundreds of liters of water, letting us collect anything too large to pass through. We haven’t been able to identify this species yet, making it a bit of a mystery. But the bigger mystery is still to come because this little creature is about to undergo that most universal and unknowable experience of all, death.

Death comes to the microcosmos in many forms. Like this Stentor Polymorphous, slowly expelling the contents of its once trumpet-like body into the surrounding environment. Or this dead larva, whose exoskeleton is now an inanimate host to two unicellular organisms.

Even the mighty tardigrade, which has survived as a species through multiple mass extinctions, is not immune to death. This is, of course, the natural order of things. Predators hunt, and their prey attempts to survive, with varying levels of success.

This is Loxophyllum meleagris, a large unicellular organism that we’ve shown before eating a rotifer. This one is practically stuffed with those multicellular creatures, we counted five rotifers inside of it. But sometimes the predator becomes the prey, and even the Loxophyllum meleagris has to find ways to ensure its survival when other species come after it.

This seemingly unlikely threat is the Lacrymaria olor. Its name in Latin means “tears of a swan”, a name that suits both its teardrop shape and its neck-like extension, which gets up to eight times longer than its body in search of prey. Sometimes, we can see its neck poking out of the dirt on our microscope slide.

But even knowing that, you’d be forgiven for thinking it unlikely that something so small could pose a problem for those larger Loxophyllum. And yet, the Lacrymaria manages to take quite a chunk out of the Loxophyllum. The Loxophyllum though, survives thanks to its ability to regenerate the piece that was taken, but not all prey gets so lucky.

Here, this rotifer has been killed by a heliozoan, destined to become food, a fate that this flagellate is about to share as it becomes captured by a heliozoan that is in the middle of cell division. The flagellate has been trapped by those long extensions, called axopods, that radiate out from the heliozoan’s body. As the flagellate comes further in, it will be engulfed by the cells into its own food compartment called a vacuole.

There, it will be lysed open and its contents digested by the heliozoa. In the end though, the natural order comes for predators too. Here, another heliozoan’s dying cellular body attracts the various decomposers of the microbial world.

Aside from predators, there are many other factors that lead a single-celled organism to die, changes in temperature, oxygen concentration, pH, water quality, so much more. This single-celled organism is swollen because the water surrounding it is entering the cell via osmosis. Many organisms have water pumps called contractile vacuoles that they use to push water back out and prevent that swelling.

But as in the case of this organism, sometimes those contractile vacuoles stop working, and when that happens, the cell swells and explodes. Other times, the cause of death is harder to determine, like this Paradileptus that spent several hours swimming before going still, its shape beginning to change until it melts away, seeming to kill not only the Paradileptus but this small green cell swimming nearby, but leaving other smaller flagellates seemingly unaffected. And this brings us back to the beginning, with our mystery organism that is about to undergo a death laden with more mysteries.

At first, the cell looks like it’s just melting away, dissolving into something that resembles a microbial Milky Way, except that for a few seconds, it almost looks like the cell membrane is able to close itself back up. We think, though we can’t know for sure, that some of the mechanisms inside the cell are still working, and that the organism is trying to recover. But alas, survival is not in the cards.

Its membrane goes through lysis, releasing its insides into the surrounding environment. This death is unlike any other kind of death we’ve observed under our microscope, and we’re still not sure what caused it. Perhaps there were so many organisms in the sample that they depleted the oxygen, and this organism could not continue cellular respiration.

But perhaps it was something else. Death at every size holds its own mysteries, but it also reveals. The observations we make, even the guesses we come up with, tell us about the way these microbes interact with their environment, the way their own bodies work, and the connections that exist between them.

It is only ever in the mysteries that knowledge is waiting to be found. So we just saw a small fraction of how many ways there are for microbes to die. But maybe now you’re asking yourself a more fundamental question: what even is death?

Well, weirdly, none of us will ever fully know the answer. But that doesn’t mean we can’t try to use what we know of chemistry and life to begin to describe it, as we’ll see in our next video. This is a ciliate, Loxodes magnus.

It is about to die. Of course, depending on your time scale, we’re all about to die. To the grand canyon, or the sun, things that have existed for millions or billions of years, we are each weird little bubbles of peculiar chemistry that form and then pop, form, and then pop.

But this ciliate, and with our new microscope you can really see those cilia beating, is about to pop right before your eyes. It looks fine right now. You can even see, inside it, it’s last meal, a Trachelomonas.

So we don’t think it’s starving to death. It seems to be trucking along just fine. Loxodes Magnus are microaerophilic organisms, preferring a low concentration of dissolved oxygen in their environment, but not too low.

So maybe the concentration on the slide was too high, though we’ve witnessed many others who have been just fine in our preparations. So no, we can’t tell you why this ciliate is about to die, but we can tell you that right here, that’s where James, our master of microscopes, first saw something strange. The moment the ciliate shifted direction, a little trail of cell membrane and cytoplasm.

No reason. Nothing grabbed it, it didn’t snag on anything. But a little bit of what was once a part of the organism was suddenly, no longer a part of it.

That cytoplasm is full of complicated molecules that are what chemists would call, far from equilibrium. Equilibrium is the situation in which chemicals no longer have a tendency to react over time. In general, a thing that you can say for sure is that all the stuff outside of living cells is either at chemical equilibrium, or it is headed there.

Whereas stuff inside cells is not at equilibrium, and it’s not headed there either. How are all of these chemicals that, if left alone, would rapidly reach equilibrium managing to not do that? Life.

That is what life is. A bunch of chemicals that take in energy in order to keep each other from reaching equilibrium. Quick break from our friend, the way we define life in biology classes is, wrong.

It’s not even really a definition, it’s a set of qualifying factors. Life has to take in energy. Life has to reproduce, it must respond to its environment, it must consist of cells.

This is not a definition, it’s an attempt to draw a line, to create a boundary. And that makes sense for things that are actually amorphous and complicated, like social constructs. But life is not a construct of our opinions, but of reality.

Life is a chemical system that uses energy to keep itself from reaching chemical equilibrium. Why do they do it? Oh, well maybe let’s not go that deep, at least not today.

Suffice it to say, a system that did this developed on this planet and now, billions of years later, it is still doing it. We have many things in common with this ciliate, and not to belabor the point, but one of those things is that we will die. You’ve may have noticed by now that this video isn’t about what life is, it’s about what death is.

It’s just that, first, we had to define life. Life is chemicals working together to take in energy to keep themselves far from equilibrium. Death is not the return to chemical equilibrium.

The process of decay can last decades. Likewise, many parts of my body will return to equilibrium over the course of my life, I’m shedding skin cells right now and so are you. The atoms and molecules of my body are replaced with new ones over and over and over again.

But I will only die once. Likewise, our ciliate has been shedding cytoplasm and cell membrane for minutes now, and that shed cytoplasm is dead, no doubt. But the organism lives.

Its chemistry continues. For now. Death is the moment when the system that maintains the far from equilibrium state ceases existence.

And we can imagine that at many scales. That can happen to individual bits of an organism, as it is happening to the chemicals spilling out of our Loxodes right now. It can also happen to an individual cell in an organism.

And that happens all the time. It is happening right now inside you. It can also happen to an organism.

That’s what we usually think of as death, with our focus, so often, on the individual. But we can keep moving up the scale and find yet other kinds of death. When a common genetic system that was useful for keeping many similar but individual organisms alive ceases to exist, that is an extinction.

A kind of death. And when the system that has kept all life on earth far from equilibrium for billions of years, that system that we all share of nucleic and amino acids, when that ceases to exist, that will be something else. A terrible kind of death that we do not even have a name for.

But it will be a death. The largest death, I suppose, until heat death, when everything in the universe has found equilibrium. Our ciliate is about out of time now.

I don’t know when we can call it, when we can pronounce the time of death, but this seems as good a time as any. Here, we have death. The system that was using energy to keep itself from reaching equilibrium has ceased to exist.

Hey, welcome back. If you’ve come out of that video with some existential dread about the state of the universe, that is very reasonable. However, on our next stop in this journey, we’re going to argue that sure, chemical equilibria are scary, but if you’re a nematode, maybe you should worry about fungi first.

There are plenty of horror stories that begin innocuously enough. A new home, a camping trip with friends, a doll purchased at an estate sale…. This one starts with some ponds, the same set of ponds that James, our master of microscopes, has been sampling every week for the past three years.

Which means that he’s collected so many microbes from these waters that you might think they’d get a bit boring or redundant. But you should never underestimate nature’s capacity for surprise. Recently, James came home with some samples from these ponds.

And as usual, he prepared some slides and checked on the organisms within, finding some nematodes like this one slithering about on the slide. And all seemed well, so he stored the slides and his new friends in a humidity chamber and waited to observe them after a few more days. But two days later, all would not be well.

This is where we build our suspense. In a movie, this would be the moment where we assess the unsettling basement or the dark woods, and then consider retreating to safety. This is the creepy doll, only there hasn’t been any thumps in the middle of the night, so everything seems okay, right?

We’re looking at the spores of a fungus, one belonging to the group Arthrobotrys. And when it’s just floating around like this, it seems quite harmless—especially when compared to the nematodes we showed earlier, which are part of a whole family of worms that are notorious for their parasitic lifestyle. And if you were to write off Arthrobotrys as a potential threat, you would be correct… most of the time.

It does spend much of its life aligned with the dead, but only to sustain itself on the remains of decayed life and organic matter. Arthrobotrys species are found all around the world, occupying everything from soil to animal feces in the many varied climates that make up our planet. And wherever it is, the fungus ensures that nutrients like nitrogen from dead organisms and other waste cycle through ecosystems.

But when nitrogen is scarce, these fungi will resort to hunting it down from living sources. And what better prey than the nematode, a fellow dweller of the soil and one of the most abundant animals on earth? When James put his slides into the humidity chamber, he had no notion of what these nematodes would be facing, and so no expectation of what he would find.

But when the slides came back out, what he observed was something he’d only seen once before, in a drawing done two years ago by one of his close friends, Katelyn Solbakk. In it, you can see a nematode whose body has been clinched into segments by some kind of bulbous, thing. What you’re seeing is the fungus’ most brutal design.

But to get there, it must morph from decomposer to predator, no longer consuming what has already been dead, but actively killing. It begins by weaving a trap out of itself. It threads the hyphae of its mycelium out and then back in, forming a living loop that repeats to form a net.

But a net is only one part of a trap, the other part is the lure. The fungi can find nematodes by following traces of their pheromones like they’re breadcrumbs. And more nefariously, they can mimic the smell of certain food cues to draw the worm in, like a siren working through scent instead of song.

The nematode has no reason to suspect anything, even as it swims closer and closer and eventually through the fungal rings. But as it does, the movement of worm and water triggers the rings to constrict. The worm is trapped, but the worst is still yet to come.

The fungus’ hyphae begin to grow off from the loop, puncturing the worm’s cuticle and paralyzing it. The threads swell up into a bulb that produces more hyphae to spread through the rest of the nematode. And then the fungus feeds and feeds, quickly digesting the rest of the nematode’s body from within.

It is a gruesome death. Here is one nematode, just recently trapped. And here is the worm again, four days later.

You can see the infection bulb where the fungus first punctured and expanded. And the whole body of the worm seems taken over, no longer a clear tube, but instead a corpse that has become home to its cause of death. The Arthrobotrys fungi are not the only ones capable of trapping and feeding upon nematodes.

There is a whole range of nematode-trapping fungi with their own methods, though the species Arthrobotrys oligospora is perhaps the most plentiful of these fungi and also the best studied. Maybe it’s just us, but it’s somewhat unsettling to realize that this insidiousness is all the work of a fungus, a thing that can seem so inert compared to the wiggling, active worm that it targets. But fungi do have a kinship with horror stories.

Their frequent role as decomposers naturally connects them with the dead. Plus, they come equipped with their own creeping sense of dread with images of mycelia weaving through bodies. And authors have drawn inspiration from the notion of fungal horror.

There are many works--like the famous Gothic tale We Have Always Lived in the Castle, or the short story “The Voice in the Night,” or recent novels like Mexican Gothic and Wanderers— that draw on everything from poisonous mushrooms to colonizing fungi to create their terror. But whatever we seek to scare ourselves with in fiction, horror has its purpose in nature. As we’ve pointed out, nematodes are one of the most abundant animals on earth.

They play an important role in decomposition...but they’re also the source of many diseases—both in animal bodies and in plants. So having them be slightly less abundant is important to our ecosystem as well. In fact, scientists have been studying these fungi to develop better nematode-fighting strategies for agriculture.

So as is the case with many good horror villains, there is a version of this story where the nematode-trapping fungus is the hero. Unless, of course, you’re the nematode. And for our last video, our microbes are dying at the hands of an unusual enemy.

It’s James, with an UV laser, in the laboratory. Maybe it sounds like a microscopic version of the game Clue, but there’s a point to it all, we swear. Blepharisma have appeared on our channel several times before.

In fact, this channel got its start thanks to a video that James, our master of microscopes, once posted of a Blepharisma dying. Around three million people watched that video, including me, your host Hank Green. So if you enjoy this channel, you can thank that dead Blepharisma.

But perhaps you should wait for another day to thank them. Because in about ten seconds, you’re going to watch a Blepharisma explode. Here it is, glowing with autofluorescence underneath UV light.

You can see its oblong shape and oral groove outlined in red…but not for long. The red becomes brighter and brighter, but it also looks like it’s starting to expand. And then suddenly, the walls of the blepharisma burst, the organism popping like a crimson balloon.

The blepharisma bubbles and pours into its surroundings and it all happens within a matter of seconds. Let’s watch it again. Dead or dying microbes are a common enough sight in our journey through the microcosmos.

And there are many potential culprits behind these deaths: predators, accidents, environmental changes, the inevitable march of life into death. But the culprit this time… well, it was us. Us and the UV light that is part of our new fluorescence microscope upgrade.

And our UV light has been very exciting for us. In particular, it’s allowed us to look for methanogens, or Archaea, which sometimes take up residence inside protists. Under normal light, it’s hard to tell the tiny archaea and the tiny bacteria apart.

But under UV light, the archaea will shine blue. So UV can reveal new aspects of the microcosmos. But if you’ve ever fallen asleep on a beach or just stayed out in the sun a bit too long, you may have also experienced the darker side of UV light.

No one wants a sunburn, but fortunately, we have defenses, like hair, and melanin, and sunscreen which can block or absorb UV rays before they cause further damage in our cells. We also, and this is crucial, have more than one cell...so if some of them die, which when you get a sunburn they do, the rest of our bodies can live on. Not all organisms have these sorts of protections.

Or if they do, they’re designed for exposure to the sun, not the intense scrutiny of our UV light. So when James wants to hunt Archaea, he has to be careful. He can quickly shine the UV light to see if anything blue appears.

But he has to quickly shut it off. Because as we’ve seen, even a few seconds of exposure to the UV light will kill off his pond buddies. We want to note that as we said earlier, death is a common reality of the microcosmos…we just usually prefer to walk in on a microbe dying rather than being the cause of death.

But for this episode, we decided to make an exception and use our UV light for an extended period of time, with the knowledge that it would kill the microbe we were watching. Because these explosions illustrate the cost of doing business with light. The word for this business is phototoxicity.

Death by light. And while it can happen under other monochromatic lights, the particular wavelength and intensity of our UV light makes it much more harmful to our organisms than our other red, blue, or green light sources. This death starts with excitation.

When the light hits the organism, it can potentially excite chemical structures inside the cell, sending electrons up and down, and producing fluorescent colors in the process. But the colors aren’t the only thing that gets created. If there’s oxygen around, it will react with the excited fluorescent molecule, creating what are known as reactive oxygen species.

In biology, reactive oxygen species are byproducts of different cellular processes that metabolize oxygen, which can make them part of normal life. There are even reactive oxygen species that are involved in signaling pathways. But the “reactive” in their name is key to what makes an excess amount of them dangerous.

If you are an organism, and you are, there are a lot of reactions you want to have happen in your cells. You want your DNA to link together correctly, you want your enzymes to find the right substrates. But reactive oxygen species are happy to react with all of those molecules too, damaging them and getting in the way of the chemistry that we need to survive.

What phototoxicity will look like depends on the organism and the light being directed at it. For the organisms we’ve been showing here, like this homalozoon, the overall effect of this intense UV light seems to be unanimous: the cell swells up and bursts open, like a galaxy erupting on our slide. But while the overall effect is the same, the internal machinations are likely different, triggered by a complex interplay of different chemicals that nonetheless react to our light source in a similar, catastrophic fashion.

While we’re not sure of the culprits behind the homalozoon’s death, we can identify one of the chemicals that likely sets off the blepharisma’s death. It’s the reddish pigment molecule called blepharismin that gives the ciliate its color under more normal circumstances. Outside of the UV light, you can see the membrane-bound pigments neatly distributed along the rows that stretch from one end of the blepharisma to the other.

But under our UV light and with oxygen in the environment, the blepharismin reacts to form reactive oxygen species, and death follows quickly from there. But while toxic in our experiment, we should note that the blepharismin serves a key purpose for the blepharisma: defense. These pigment molecules are toxic to some of Blepharisma’s predators in both the light and the dark.

That makes the pigment somewhat like UV light: necessary for survival, yet also a delicate negotiation. But in the same way that we manage our relationship with the sun, scientists have learned ways to manage these phototoxic reactions. They’ve had to in order to understand how we can use fluorescence microscopy to study cells and organisms.

They’ve learned how to modulate wavelength and intensity and duration, along with many other factors, to wield light in a way that better serves their purposes. In the case of the blepharisma, for example, scientists found that using a moderate light for around 1 hour wasn’t much of a problem for them. But with more time under the light, the cells would eventually die.

It’s easy to think of the microcosmos as a separate world from us, even when we know that the microscope is a bridge between large and small. But these deaths at the hand of our supposed bridge are a cautionary sign that we are encountering microbes in a world that is both natural and manufactured at the same time. The way that we light that world impacts the way we see the organisms, and it also shapes their lives—reminding us that they are stronger often than we can fathom, but fragile nonetheless.

And that brings us to the end of our tour of death in the microcosmos today, an end to a story of ends, you might say. But maybe what we’ve seen today is that there really is no end, is there? Just pauses on individual stories that nonetheless endure in the remains of the world left behind.

This channel would not be what it is if it weren’t  for one very key invention: the microscope.   Everything we see, we see with the aid  of light and lenses, expertly deployed   by our master of microscopes, James. And if you’ve been on this journey from   the beginning, or if you’ve ever gone back to  revisit our earlier videos, you may have noticed   that things have changed a bit around here. Everything we can see is a function of the tools   we use.

And as we’ve explored new microscopes  along with other new tools and techniques,   that means we’ve been able to see the microcosmos  differently. Colors have changed, contrast has   deepened, and sometimes things even sparkle. So today, we’re tracking our journey… through   our journey.

We have compiled the videos we’ve  made that focus on how our microscopes work   and the ways we have improved our methods over time  so you can see how much of microscopy is an art—an   art that uses light in incredible  ways that only physics could predict.  So to start, let’s go back to an  early video, where we went through   the different methods we used at the start of  our channel to illuminate our samples. Think   of it like an introduction to optics, told  through the bodies of tardigrades and algae. This is a ciliate, just a eukaryotic microbe  waving its cilia around under our microscope.

This is the same ciliate. And yup, here it is again… …and again. But as you’re probably noticing,   while the rough outline of this organism seems the same from shot to shot,   the ciliate itself and the world around it clearly  look very different from shot to shot.

Colors   change, details are more apparent. In one  case the organism seems lit from within. On this channel, we're constantly flipping between  different ways of capturing images of organisms.   So which one of them is what they  actually look like?

Well...none of them. Anything you see through a microscope is an image  —which in our case, means that everything we   show you on this channel, every frame, is not the microbial world itself.   It’s an interpretation of life on the other  side of our objective, translated through the   lens into details, shapes, and colors—all affected  by the way we light up the life we want to see. Light is amazing, it’s also very weird.  It travels in waves, and as it interacts with   particles and materials, it scatters, shifts.

Even  if we can’t actually see those light waves in motion,   so much of what we observe in the world  around us is rooted in the physical properties   that define those waves—like how we can observe  certain frequencies of light as colors. But waves   have far more to them than  just their frequency, and microscopy   has combined the resourcefulness of many  different sciences to use light to give us   different ways to peer into the microbial world. So let’s start simple with the good old fashioned   white light.

Early microscopists used oil lamps  and sunlight to see through their microscopes,   and while the technology has changed, the  simplicity of this has endured into   the modern technique of brightfield microscopy. It all starts with a source of light, though   modern microscopes have their lamps built in, set  up underneath the stage that holds our sample.   Light travels from the source through  a condenser, which works to focus   the light onto the sample above it. This focused light travels through the   sample towards the objective lens, which takes  in an image and magnifies it into these bright   backgrounds with organisms, sometimes rendered  transparent by the intensity of the light.

You might say that this is as close as we get  to seeing what the microcosmos actually looks like,   but that would be like taking 2000 watt  light bulb into your living room and saying,   “this is what my home looks like.” Light affects  things, and we are not shining light on these   organisms, we’re shining light through them. Now, brightfield might seem relatively simple,   but that simplicity has been incredibly powerful  in allowing scientists old and new to wade through   microscopic waters. Still, there are limitations  to consider with any scientific technique,   and one of the major challenges for  brightfield microscopy, particularly   when we’re looking at microbes, is contrast.

Pigmented organisms are easy to visualize against   the bright background, but in cases where  the organism has been rendered transparent,   it can be harder to distinguish their bodies  from the rest of the world they inhabit.   Scientists can navigate these challenges using  stains that make certain structures more visible,   but for our purposes, we like to avoid stains  because they can affect the microbes themselves. There are ways to contend with this challenge,   one of which is built on one of those  simple-yet-strange properties of our world:   you don’t always need to shine light directly  onto an object to see it. This technique is called   darkfield microscopy, which sounds like it  must be the opposite of brightfield microscopy.

It’s not. The two techniques  are actually very similar:   light travels from a source through a condenser,   interacts with the sample, and then travels into  the objective lens, producing the image we see. But what we want in darkfield microscopy is for the  beam of light to hit the sample, but not our eye.   In darkfield microscopy, a circular  disk is placed inside the condenser,   blocking the central part of the light light from  shining through the sample and into our eye, or,   our camera.

This means that when there is no  sample on the slide, all you’ll see is black.   But the disk doesn’t block all light: there  is still a hollow cone of light that travels   around the disk, unable to reach the objective or  our eyes, but that light still hits the sample.   When it does, their microscopic, transparent  bodies scatter those hidden rays into our view. And as they do, an image of their  bodies forms against a dark background,   providing us with this almost cinematic footage. Another method to get better contrast than   brightfield microscopy is called  phase contrast microscopy,   and it’s built on working with a property of  light that we can’t actually directly experience.

Microbes , or really anything, that is easily  visually observed with brightfield microscopy   are called amplitude objects because  as light passes through them,   the amplitude of the light wave changes,  which we see as changes in light intensity. But there is another class of specimens: these  are called phase objects. As light passes through   these objects, the waves slow down and shift  slightly in phase compared to the unaffected light   around it.

And if you’re wondering what that means  in terms of what we can see, that’s the issue:   our eyes don’t process these differences  in phase. And so in the final image,   these objects, or in our case,  organisms, are very difficult to see. In the 1930s, a physicist named Frits Zernike  developed a method to shift the direct   light just slightly enough so that these  changes in phase could actually be translated   into changes in amplitude, producing  an image of these formerly hard-to-see   phase objects by essentially treating them  as amplitude objects.

There is a lot of   physics to this that we are glossing  over, but the result was so important   that it would eventually win Zernike the  Nobel Prize in Physics. And of course,   selfishly, we appreciate his work because it lets  us see more of our more hidden microbial friends.   And for the last type of microscopy we’ll go over  today, we’re going to be getting into another   property of light that we can’t directly  see, but that makes the microcosmos glow.   Most of the light we see has an electrical field  that vibrates in all sorts of planes relative to   the direction the light is traveling in. But  that vibration can be restricted to one plane,   and when that happens, the light is said to  be polarized.

We[8] can’t see the difference   between polarized and unpolarized light.  You might notice the difference in how the   world looks when you’re wearing polarized  sunglasses, but these are changes brought   about by changes in color or intensity,  not the polarization of the light itself.   So when does polarized light help us  in microscopy? Well, a lot of materials   stay the same optically-speaking, no matter what  direction you shoot light at them with. But there   are certain materials where specific properties,  like how fast light travels through them, can vary   depending on which way the light is striking them.  These materials. called optically anisotropic,   can also take in a ray of light and  divide it into two separate beams.   By aiming polarized light at our sample  and then reconstructing an image based on   how the various parts of the organism  interacts with that restricted light,   particularly by how it might cause the light  to split, we can see more of these optically   anisotropic materials in action.

In our case,  it often takes the form of shiny crystals.   So we’ve given you the big overview of  what these different techniques can do,   let’s go back and review what that means  for the original ciliate we started with.   This is the ciliate under brightfield microscopy.  The background is bright, the image produced by   changes in light amplitude that allow us to see  the overall shape. But some of the detail is hard   to make out. Under darkfield though, the contrast  increases and some of these details become more   obvious, displaying compartments and cilia in  greater detail that are also apparent under   phase contrast.

And then under polarized light,  the crystals that blended in with the organism   previously are now quite visible and vibrant. There are, of course, many other microscopy   techniques, but we think it’s incredible that  with just these four, the world of the microcosmos   looks almost like different universes,  wrapped up in one invisible world   around us. The journey, it seems, is not  just about what you see, but how you see it.

And ultimately, none of these views are  what the microcosmos actually looks like,   either that, or all of them are. Our brains play  tricks on us to make us believe that the world   looks one way, but the world looks different  at night than in the day, and both of those   things have more to do with the physiology  of our eyes than their objective reality. Asking what microbes look like is, to  some extent, forcing our own experience   onto something that is beyond it.

Which is  not something I ever would have thought of   if it weren’t for this little YouTube channel. Now we have been really fortunate over the years  to have the support of so many people,   and to be able to direct that support  back to the microscopes we use.   In 2020, we introduced James’ new microscope,  which he was able to buy thanks to crowdfunding.   This was our first major microscope upgrade on  the series, and it’s incredible to see the jump   in quality and how it compares to the different  techniques we showcased in the previous video. This golden algae is swimming around  in water and white light, illuminated   from below using a brightfield microscope.

And here we have that mallomonas sample,   filmed a day later using a different  technique thanks to a brand new,   freshly installed microscope. When James, our master of microscopes,   saw them for the first time, he got a little  emotional because, to quote him directly,   “the thing I know so well, was looking  so so much different than usual.”  Those differences in how the mallomonas look are  due to a type of microscopy called “Differential   Interference Contrast.” Or…DIC. Earlier this year, James started   a crowdfunding campaign to purchase a  new microscope, one that would allow him   to use differential interference contrast  to produce these images you are seeing now.  Now technically, “differential interference  contrast” is not a type of microscope.

Rather,   it’s a method that enhances contrast. And as you  can see, the final product is an image that seems   almost 3-dimensional. Just watch this stentor  coeruleus as it swims across the slide.

Its   cilia are so prominent that  it almost feels like you can   touch their vibrating fuzziness. And  the striations down its body are so   sharp that when the stentor contracts, it’s  like you can feel it pushing inward and then   back out again. I don’t know about you,  but it’s almost like you could reach out   and touch it—like you’re in a movie theater  with 3d glasses on, reaching out to something   that seems to be reaching back out to you.

Differential interference contrast microscopy   was invented by Georges Nomarski in 1952,  building off the principles underlying phase   contrast microscopy, which is a different though  similar thing. But both of these methods work   to translate invisible shifts in light phase that  can happen when studying certain samples into   visible changes in light amplitude, which we see  as changes in light intensity. And as a result,   both techniques enhance contrast and let us see  parts of the microcosmos that might be invisible   with regular brightfield microscopy.

But phase  contrast microscopy and DIC shift and work with   light in different ways to accomplish this goal. How do they do it? Well, if you’ve ever taken an   optics course, you will remember how terrible  that was.

But, very basically, phase contrast   and DIC are two ways to take advantage of  a strange reality. Light actually travels   further through some materials than others. Kinda.  The optical path length is a function of both   the distance between two points and the refractive  index of the material the light is travelling   through.

Basically, denser samples have longer  optical path lengths. Phase contrast microscopy   takes advantage of that, making areas with  longer optical path lengths look darker.  DIC on the other hand, doesn’t make areas  that are denser darker, it uses some very   cool optics to create sharp contrast in areas  of rapid change in the optical path length.   So, the faster the gradient from more to less  dense or vice versa, the more contrast you see.  But it’s also important to note that  while these images are the result of light   traveling through a sample, they are not  an actual topographical map of an organism.  DIC microscopy is a technique, but it does require  certain physical additions to a microscope. For   one thing, we need to split light apart.

So,  first we have a prism that splits polarized   light into two orthogonal rays of light that will  then pass through the sample and interact with it   in different ways. And then there’s a second  prism that recombines those two rays after   they’ve traveled through the sample, forming an  image based on the differences those two beams of   light experienced for our eyes and cameras to see. In addition to these prisms, this technique relies   on having higher quality objectives than what  we’ve been using in the past.

The objectives are   the little lenses at the bottom of the microscope  that actually do most of the magnification. Not   many people would realize this, but the objectives  we use for Journey to the Microcosmos are some of   the cheapest and most common types of objectives,  they’re known as achromatic objectives. But while   these objectives produce so many of our favorite  images, James needed a microscope update to be   able to do differential interference contrast.

So with help, perhaps including some from you,   James bought a new Zeiss microscope. It took eight  weeks for it to arrive, which is a long time to   wait. But luckily the manufacturers sent him a 150  page manual to occupy his time until it arrived.  When the microscope finally showed up, James still  had to wait for an engineer to install the nearly   50 pound machine.

But he didn’t waste any time.  James went out to gather his precious Mallomonas   and prepared a slide that night, keeping it stored  in a humidity chamber so that he could look at it   as soon as the microscope was ready. The next day, the Zeiss engineer took   3.5 hours to put all the pieces together  and set everything up. But from there,   well, it was all on James to observe what he  could and experiment with his new microscope.  Some of his experimenting has been geared towards  making sure that he can get the best image   possible.

These objectives have a much smaller  working distance compared to our previous ones,   and so you need to prepare the slides  a lot thinner to get a sharper image.   So even for our master of microscopes,  there was somewhat of a learning curve   to make sure he was getting videos that  were as sharp and vivid as he wanted.  But some of the experimenting is  based out of that most exciting thing   of all: curiosity. He’s removed some of the  prisms to see how it affects the footage,   and added a magnifying glass over the light  source to scatter the light in different ways.  That’s the joy of the microcosmos. It is so  infinite.

There’s it’s own objective existence,   full of so many more organisms than  we’ll ever be able to identify.   But there’s also the infinite  nature of how we experience it,   of how our own view of this world is shaped by  the many different tools we use to observe it.  And maybe that’s the case all  the time...that what we see   is as much about how we view the world as it is  about the world itself. The same thing with a   different lens, a different technique, a different  base of knowledge, can look completely different. As we start our third season of Journey to the Microcosmos, we hope you will continue to join us through both our new and old lenses, as we uncover more of this hidden, unending world.

In 2021, James went all out on improving his  toolkit, investing in higher quality objectives,   a polarizer, and the equipment he needed to do  fluorescence microscopy. If you’re not sure why   those things had all of us on the Journey  to the Microcosmos team so excited, well,   that’s what our next video is for. You will get to  see just how much of a difference an objective   can make, and why electrons jumping up and down  can reveal a whole new side to the microcosmos.

We’re nearing the end of our fourth season of  Journey to the Microcosmos, and over that time,   we’ve been able to see microbes through  all kinds of lenses—literally—thanks   to James, our master of microscopes. For  the rest of us on the Microcosmos team,   it’s been really cool to not only see  the footage that James has recorded,   but also see the journey that he has  been on with the microscopes themselves.  Now, we’ve been able to make this journey  because of all of you and the support you   have shown to this channel by watching and  commenting and sharing with others, or even   and especially by supporting us on Patreon.  We’ve all been on this journey together, so   let’s look back on where it’s taken us, and also  get a peek at where we’re going in the future.  In the first season, James started out with  two microscopes. One was a microscope that   he assembled himself from different parts for  under $200.

The other microscope was a Motic   BA310 that he received from a microscope  company shortly after we started this channel.  And these microscopes give us so much beautiful  footage, thanks in part to the different   techniques that James used with them, whether that  was bright-field or phase contrast or darkfield,   or some other technique to manipulate  the light shining on the microcosmos.  But these microscopes also had their  limits, so we were really excited to see   what would happen when James bought a new  microscope before the start of our third season:   the Zeiss Axioscope 5. This new microscope  came with higher quality objectives (which   are the lenses on the bottom of the  microscope), as well as prisms that   made it possible for us to look at microbes with  differential interference contrast microscopy,   a technique that sharpened their features and made  them seem more three-dimensional on our screen.  We have videos exploring these different  techniques and microscopes, so if you want to   learn more about them, we recommend checking those  out because light is much weirder than it seems.   But we wanted to do a quick summary for reasons  that are probably very clear from the title   of this video: we have upgraded our microscope! The Zeiss Axioscope 5 was a very exciting change,   but it was also an expensive one.

So when James  bought it, he was only able to get two objectives:   a 20x objective and a 63x objective. Adding that  to the 10x magnification the eyepiece gives,   let us magnify our samples by 200x and 630x  respectively, which is not bad. But a standard   microscope usually comes with 5-6 objectives for  a reason: the microcosmos is full of things both   small and large to observe and there  isn’t a one-size-fits-all objective   that you can use to observe everything.

So one of James’ priorities was expanding   his set of objectives so that he could have  the flexibility to see the microcosmos at more   scales. And thanks to his recent upgrade,  he’s now gone from a set of 2 to a set of   six that range from 5x to 100x and that gives  us a magnification range from 50x to 1000x.  Since I have gotten my own prototype  of the microcosmos microscope that   we are manufacturing thanks to backers of our  kickstarter, I’ve been amazed at how important   it is to have this variety of options, and how  much time I spend on the lower magnifications   and well as going deeper . So without further ado,   let’s introduce these different objectives by  looking at a ciliate called Tetrahymena that’s   taken up residence in a rotifer exoskeleton.

With the 5x objective, we can see   little dots swimming in what used to be empty  space, but it’s hard to make out the details.  When we move to the 10x objective, we can  start to make out shapes of the Tetrahymena   and more of the emptiness around them,  but it’s difficult to see the exoskeleton.  Then as we start to go up in magnification,  the details of the exoskeleton become clearer.  It's cool to see more and more of those details  as we zoom in. And especially when we get to   the 100x objective, which gives us 1000x  magnification, you can make out so much of   the insides of the Tetrahymena, that they  look like bags of very resilient bubbles.  Here’s another ciliate called a Pseudoprorodon  that James looked at through all of these   objectives. At first it just looks like a little  green thing floating around and knocking into   stuff.

But as we go up in magnification, we can  start to make out the subtle undulations of its   body, which shifts the different shades of green.  And as we look even closer, the green itself takes   on a new life, revealing the perimeters of the  individual algae inside the pseudoprorodon.  Revealing those details inside the tetrahymena  and pseudoprorodon comes at a cost. We can’t   see as much of the world around them,  whether that’s the emptiness around the   rotifer exoskeleton or the objects that  the pseudoprorodon keeps bumping into.  Whether or not you need those details depends on  what you’re trying to learn or what we are trying   to show, and that’s why having the flexibility  to look through a wider range of objectives   is such an exciting upgrade for us. Objectives are  our gateways to the microcosmos, and we’re excited   to have all of these new scales to look at.

We’ve also made some other upgrades so we can   revisit techniques we used on our older  microscopes. Like a polarizer, because   who doesn’t love this shiny, iridescent stuff? But we’re not just sticking with what we’ve   known.

Thanks to this upgrade, we’ve got a  whole new way to look at cells: fluorescence   microscopy. And we are so excited for this one. The techniques we’ve highlighted so far   are all clever manipulations of white light, a  mixture of many visible wavelengths of light.

But   in fluorescence microscopy, instead of  illuminating our sample with that mixture,   we choose a specific wavelength of light. That light is called the excitation light   because, well, it excites certain  structures based on their chemistry.   And those excitable structures then emit some  light of their own. The emission light will   then travel back through the objective, get past  barriers that filter out the excitation light,   and then show up in our eyes as vivid color.

With our upgrade, we have four different   wavelengths of light that we can  use for fluorescence microscopy,   and the parts of the sample that we see  fluoresce will depend on which light we use.  In this footage, we’re looking at the  Pseudoprorodon that we saw earlier, only   this time it’s being hit with 385 nanometer light.  The red we see is the fluorescence from natural   structures, this is called autofluorescence.  The rest of the organism is dark because   it did not respond to that 385 nanometer  light, making the color all the more vibrant.  But the power of fluorescence microscopy is  not just finding fluorescent structures that   exist in nature. Chemists and biologists  have developed all sorts of ingenious   techniques to highlight different parts of  cells using fluorescent dyes and proteins.  James is working on building up his arsenal of  fluorescence microscopy tools and techniques   so that soon, we’ll be able to dive even deeper  into the microcosmos to seek out things that have   been hidden from us before, uncovering the unseen  world inside the unseen world that surrounds us The upgrades we’ve talked about in  the last few videos have involved   fancy equipment and exciting techniques. But light   is so weird and powerful that sometimes you don’t  need anything particularly fancy to make it do   something useful.

Sometimes you just need  the right objective and a little bit of oil. If you’ve been following along with us lately,  you probably know that we’re pretty excited   about our latest microscope upgrades. And can  you blame us?

It's like having a new toy that’s   actually a spaceship. It’s pretty hard to  shut up about something that is shiny and new,   and also takes you into a whole new world. And that’s kind of what it feels like to see   our familiar friends at this magnification.

Even  though we’ve seen tardigrades many times before,   in so many different ways, we  have not seen them like this.  And bringing this level of magnification gives  us a new layer of the microcosmos to explore.   It’s like we’ve descended into a new level of  a cave, getting further from our world—though   fortunately for the easily scared among  us, the cave is metaphorical and the risks   are negligible. All we need for the journey are  some objective lenses and a tiny drop of... oil?  Yeah, oil. But to explain   what that oil does, we’re going to have to  get a little technical first.

Because yes,   we love our microbes, but we’re only able  to see them thanks to light, and light   is complicated, but we need to give it its due. So let’s start with the objective lens.  The main job of the objective lens  is to take light leaving the sample   and focus it into an image for the us  to see. And there are two main numbers   that describe the objective lens.

The first one’s  pretty obvious: it’s the magnification. But the   magnification you see written on the objective  lens is not the same as the magnification we   write up on the corner of the videos. That’s because the objective lens isn’t   the only source of magnification in our  microscope.

We’re also looking through an   eyepiece that has its own magnification. So the  final magnification that we see is the product of   the objective lens’ and eyepiece’s magnifications. Now we don’t usually do math on this channel,   but this is one of the rare cases where  the math and optics is straightforward.  In our microscope, the eyepiece has a  magnification of 10x.

So if we’re looking through   a 63x objective, the final magnification is 630x.  And that is the number you’ll see in the corner.  But you should never trust an optics lesson that  seems simple. How could it be when the image we’re   seeing is the product of so many things at once,  some tangible like a lens, others much less so.  And what is less tangible than  light, which is, as we’ve seen before   on this channel--a very strange thing. We  cannot touch it, but we can manipulate it,   using what we know of the way it travels and  bends and reflects to see our world differently.  But light… has its limits.

The thing about  making things larger is that at some point,   it’s not enough to just zoom in. You need to  be able to capture the detail that’s there.  And this leads to the second important  number you’ll find on an objective lens:   the numerical aperture. Unlike magnification,  “numerical aperture” is probably not super   self-explanatory.

What the number describes is  the ability of the objective to take in light.  As light travels from the sample and to  the objective, it radiates outwards like   a cone. The higher your numerical aperture, the  wider the cone of light that's going into your   objective. And that allows for more rays to enter  the objective from all sorts of different angles,   helping to illuminate more details and give  greater resolution to your final image.  But of course, that’s still not all that goes  into capturing the perfect microscopic image.   Let’s take a look at this glaucoma spinning around  at 1000x magnification.

It’s visible, and you can   see some of the details. It looks…fine. But let’s take a look at it again here.   Same glaucoma.

Same objective. Same level  of magnification. What was “fine” before   now just looks dull in comparison to the image we  were seeing before with the detailed striations   and vivid pockets of green on the glaucoma’s body.

To get from one of these images to the other,   James—our master of microscopes—didn’t change  any of the technical settings on his microscope   or pull some kind of video editing wizardry. He  just simply added a drop of oil to the coverslip   encasing the glaucoma, and dipped  the objective into the oil.  Now this is because the cone of light going from  the specimen into the objective isn’t just going   straight from the sample to the lens. It’s passing  through something—it’s passing through air.  Light travels at different speeds through  different materials and when it changes   from material to material, it will bend.

This  is called refraction and materials like oil   slow light down more. And that matters to our microscope   because it gives us the wider cones of light  we need to get a higher numerical aperture.  Whether we’ve got air between the sample and  the lens or oil when we’re viewing our samples   depends on the objective we’re looking through.  Our lower magnification objectives have   lower numerical apertures, and they’re meant  to be used with air. Our higher magnification   objectives have higher numerical apertures,  and they’re meant to be used with oil.  Using air with oil objectives gives us poor  images, and using oil with our dry objective   would damage the objective.

So…don’t do that. The numerical aperture and its relationship to   resolution is important because it puts a limit  on just how much magnification we can actually do   with a given objective. There’s a general estimate  if you are looking into microscopy yourself,   that you’ll get a good magnification when you’re  working in the range that’s 500 to 1000 times   the numerical aperture of the objective.

So our 100x objective and 1.3 numerical aperture   can reach a maximum of 1300x magnification  depending on your eyepiece. With our 10x eyepiece,   we get 1000x magnification, and it works great. But if we tried to get more from this set-up by   increasing the magnification of our eyepiece  to 20x, we could not actually get 2000x   magnification.

Because of the constraints  of our numerical aperture, we would just   be losing half of our image without actually  gaining more detail in what’s left to see.  But that’s what happens any time we want to  take an image: we have to make a decision   between what we can and cannot see. There are  always choices that have to be made, and details   that have to be lost. We simply cannot see all  of the world, in all its entirety, all at once.   But we can see more of it by tracking the choices  we make as we dive deeper, choices in techniques   and materials that affect what we can see and  how we see it, and enrich the story further,   even when those choices impose constraints.

At some point in history, we wanted to see more,   and lenses with their magnificent manipulation of  light have helped us do that. We’ve used them to   see light from distant stars, and to peer into the  most mundane surroundings on earth. And whatever   image has come back to us has brought the universe  closer to view, even if it’s just in fractions.

On Journey to the Microcosmos, we love microbes. We love to see them swim and wander and live out their unique lives. But we’ve also had to make peace with the fact that the microcosmos is not a peaceful utopia.

This is a world where microbes are both residents and food, which means that occasionally, we’ll have to spend our time together watching organisms, whose bodies are fractions upon fractions upon fractions of a millimeter in size, turn into vicious predators. So today, in honor of both predator and prey, we’re going to be revisiting some of our favorite videos about eating in the microcosmos. Grab a snack, and join our microbes as they snack.

And we’ll start by revisiting some of the clever strategies that microbes use to gather their prey. Despite huge differences in morphology and biological structures, all living organisms do the same three basic things; they get food, digest it, and excrete waste materials. Living organisms require energy to live.

Some produce their own food, usually through photosynthesis, we call these autotrophs, but many organisms cannot make their own food. We call these heterotrophs and they...eat. Among heterotrophic single-celled eukaryotes, food is taken into the cell in various methods but once it’s there, it's wrapped by a membrane and forms something called a food vacuole.

Then the cell flushes digestive enzymes inside the food vacuole to start the digestion. Nutrients are taken into the cytoplasm and the waste material left in the vacuole then basically fuses with the outer membrane of the cell and what's left in the vacuole is discharged into the environment In some cases, like inside this beautiful single-celled Nassula ornata which feeds on filamentous cyanobacteria, content of the food vacuole reacts with the digestive enzymes and changes color. But that process takes time.

Because each vacuole formed at a different time, they are in different stages of digestion which gives this cell it’s colorful polka dots. But how do these heterotrophic organisms get their food? Well, in spite of a remarkable amount of diversity, a lot of microorganisms use one of the same three strategies for getting their food.

Some of this will be familiar to the macro world some of it will not. One of the less familiar is filter feeding which allows larger organisms to consume suspended food particles or much smaller organisms. There are filter-feeders in the macro-world, baleen whales come to mind.

But while a whale must swim through giant clouds of small organisms, in the microcosmos, your food can come to you. Some filter feeders use hair-like structures called cilia to create a vortex that brings other microorganisms or food particles to the cell mouth. These cilia are specialized for this task and their beating creates a current that expediently and beautifully directs every nearby thing into the waiting mouth of the microorganism.

The cilia are often too small for us to see, but you can see their effects. Take a look at these Paramecia. They're consuming tiny, tiny bacteria. and you can see their cilia causing small organisms to tumble across them.

You can also see all of their food vacuoles on the inside, and if you look very carefully, you can see a new food vacuole forming, getting filled up with those tiny, bacterial cells. Some of the best and most obvious filter feeders are rotifers, micro-animals that use cilia to create swirling vortices around their mouth parts. You can see how successful this feeding strategy is by it’s belly full of algae cells.

Every time its mouth fills with more algea, it contracts to swallow the food. Now observe these single-celled organisms called stentors, they are much bigger than most other microorganisms. You can actually see them with the naked eye. and they also use filter feeding to push all of their algal food into their cell mouths.

Our second feeding mechanism, maybe the most familiar, and the most exciting is called raptorial feeding. Raptorial feeders selectively capture prey and hunt other organisms. In this video you can see Dileptus hunting.

It paralyzes one organism with the touch of its trunk-like proboscis, and then it pulls that organism into itself in a process called phagocytosis. Many of these microorganisms are armed with something called toxicysts. These are little harpoon-like structures filled with toxins and they are located on a particular part of the cell which the microorganism uses for hunting.

These tiny harpoons are then fired when they come in contact with prey organisms which then become immobilized. This is Bursaria, it’s a single-celled organism with a huge mouth, and things have not gone well for the Paramecium that is now inside it. The Paramecium dies immediately because of the toxicysts on the inside of the Bursaria, so at least it was quick.

Now get ready for some truly gorgeous footage of a micro-animal’s day going south. First Paradileptus immobilizes the rotifer with fired toxicysts and the animal is swallowed by the single-celled organism as it swims away. This is a Frontonia which is a close relative of Paramecia, but lacks the filter-feeding habits of its relatives and feeds predaciously on large diatoms and filamentous cyanobacteria such as Oscillatoria here.

Though, in this case, it turns out this Frontonia bit off a little more than he could chew. Another raptorial feeding style is called histophagy. Histophagous organisms such as these single-celled Coleps attack injured but live animals or other single-celled organisms, sucking off hunks of tissue rather than consuming whole organisms.

When they attack an animal, they enter wounds and ingest tissue often attacking in groups because their chemical sensing abilities attract many of them from a distance, like microscopic vultures. When a number of them gather in one place, it’s hard to avoid another macro-world analog...piranhas, devouring everything soft in no time at all. There is a huge variety of raptorial feeding, this is just the beginning, but we wanted to show you one more before we move on.

This is Vampyrella, an amoeba with a suitable name. It specializes in feeding on filamentous algae. First, it bores a hole through the algal cell wall and then slurps out the gooey, nutritious cytoplasm.

Our final feeding mechanism, for today at least, is diffusion feeding, in which the predator just sits in the same place, relying on the prey to accidentally make contact. This is a Heliozoa, it’s a single celled amoeboid and because of its resemblance to the sun due to the rays coming out of its cell, it’s sometimes called the “sun animalcule”. The rays are called axopodia.

These are sometimes used in locomotion and, in this case, for hunting prey. Axopodia, are cytoplasmic extensions, meaning they’re a part of the cell membrane, even though they look like they’re sticking out of it. Each one has a central supporting rod of microtubules that gives it this rigid structure.

The axopodia are coated in organelles that discharge toxins when touched, which impair or even paralyze Heliozoa's prey. After the organism is captured, those microtubules are drawn back into the cell, thus retracting the axopodia and allowing the cell to swallow the unlucky organism, or prey is just engulfed by extrusions from the cell called pseudopodia. In this video, a rotifer has been captured by a Heliozoa and it is slowly getting eaten by it.

Now this is something that happens fairly freuently, but we did capture something unusal here. While it was stuck to Heliozoa's axopodia, this rotifer actually lays its egg. But neither egg nor the rotifer is going to escape this.

Surprisingly heartbreaking. This is a Suctorian, it is a ciliate just like Paramecium and Stentor. These organisms have hair-like cilia during the early stage of their life, but as adults they develop bundles of tentacles.

Just like in Heliozoa these tentacles are supported by an internal cylinder of microtubules. The tip of the tentacles have extrusomes; these are special structures that attach to and immobilize any other ciliates that touch them. The tentacles eventually penetrate the cell membrane of the prey, and then the contents of the prey is sucked out through the tentacle.

In the clip, a suctorian has caught 4 individual Vorticella with its tentacles and is slowly sucking their cytoplasm. It looks a little like the vorticella have the suctorian surrounded, but in fact, they are powerless to escape it. It’s a dangerous world out there.

The complex chemicals created by organisms to sustain their life necessarily are useful to other organisms, as building blocks and as fuel. And so predation evolved. It’s beautiful, it’s constant, and it’s brutal.

Now that we’ve seen how the hunters of the microcosmos get their food, let’s experience the more challenging part of the predator-prey dynamic: getting eaten. For the organisms in our next video, finding themselves in the belly of the beast is just the start of a more grueling process, one that converts their bodies into nutrients. Tardigrades are famous for their capacity to survive.

If you look them up, you will be inundated with long lists of the many things scientists have thrown their way to see if the tardigrade will survive. We’ve made some lists like those ourselves. Tardigrades on the moon, tardigrades in extreme heat.

You’ve heard about them. We’ve talked about them. What else is there?

Well, here’s a surprising fact you might not have known. For certain species of tardigrades, like the one in the middle of your screen right now, you can tell the difference between a male and a female by looking at their toes. That’s right.

The toes. These tardigrades are called Milnesium, and the males of these species have a pair of claws on their feet that are shaped a little bit differently from the rest of the claws, with only one hook on each claw instead of the usual two. But after taking a closer peek at this tardigrade’s toes, we can confirm that she is a female tardigrade.

Now, that’s a fun fact for us as we watch the clip. But it does not actually do anything for the poor rotifers that are surrounding this tardigrade, who are currently stretching themselves in and out of danger. Around the tip of a tardigrade’s mouth are small bumps that we think act like little sensors.

And when those bumps make contact with a rotifer, out pops the tardigrade’s stylet—a needle that pokes out from the tubular mouth opening. That stylet pokes into the rotifer, and from there the tardigrade uses a circular structure called a pharynx in its head to create strong suction. And then, the stylet goes from functioning as a needle to functioning as a portable, built-in straw that lets the tardigrade suck out the inside of its prey.

You can see some of the bodily fluid leaving the rotifer as the tardigrade’s stylet pumps away like a beating heart. And when she is done, the rotifer remains, like an empy coffee cup. This method of eating works quite well for the tardigrade, letting her eat quickly from the prey that is directly in front of her.

But not all organisms come equipped with a weapon that lets them turn rotifers into Capri Suns. These organisms have to turn to other methods to extract nutrients from their food. This marine ciliate is called kentrophyllum, and it came to James, our master of microscopes, in a large container full of beach sand.

On a normal day, maybe we would talk more about its funny almond shape that stretches as it swims around. But today is not a normal day because this kentrophyllum is about to be overshadowed by a rotifer. Now, where is the rotifer, you might ask?

It is inside the kentrophyllum, of course. We did not catch the moment of its capture, when the toxic needles lining the kentrophyllum darted out and paralyzed the rotifer. By the time we arrived, the rotifer had already been ingested…but it was not dead yet.

You can see it wiggling and wrestling at the broad end of the kentrophyllum, causing the ciliate’s body to wrinkle and fold in on itself. But it is trapped inside of a food vacuole now, a compartment that exists to break this rotifer down. Sure, the rotifer can try and fight against the walls of the vacuole holding it hostage.

But there’s not much it can do against the digestive enzymes pouring in, or the increasing acidity of its surroundings. A food vacuole is a hostile place to be, and for minutes, its destructive tools will go to work. In the end, the rotifer will be left in dissolved pieces to be absorbed by the kentrophyllum’s cytoplasm, sustaining the organism that was the site of its last battle.

It’s a lonely death for the rotifer, with only the kentrophyllum to witness it. Though I suppose now there is also us, sharing and immortalizing that rotifer’s last moments. And if the rotifer had not died in the body of another organism, it may have ended up like this gastrotrich.

Now to be fair, they did find themselves in a very similar situation: they are both dead. But the rotifer was eaten alive, digested from the outside

while trapped in another organism. Meanwhile, this gastrotrich is going through the exact opposite situation.

It is being eaten from the inside out, by a scavenger called a peranema. Now, as far as we know, the peranema didn’t do anything to kill the gastrotrich. It was likely already dead, though we don’t know what killed it.

But the peranema is both a hunter and a scavenger. Sometimes it can be found with other peranema as they hunt down prey, in a pack of hungry microbes. At other times, the peranema scavenges.

They are notorious for being able to squeeze their way into whatever holes they find to get into a dead organism. And in this case, the gastrotrich’s exoskeleton was the right combination of available, open, and dead for the peranema, which found its way in and decided to begin helping itself to the buffet of dead tissue around it. After all, the gastrotrich won’t be needing that tissue anymore, so the peranema might as well make good use of it.

The gastrotrich’s body has likely vanished by now. Perhaps the peranema will have finished it off entirely, or some other scavenger will have joined in on the fun. Or perhaps, it will have simply faded, like the rotifer inside the kentrophyllum, its body eventually fading into the world that encapsulates it.

But for this last moment, let’s remember the gastrotrich in a more beautiful moment under a fluorescent light, glowing in the glorious purple autofluorescence of its remains. If you think that microbes are the only organisms that eat microbes, think again. We humans have a particular fondness for one specific fungus called Saccharomyces cerevisiae, but you probably know better as “yeast.” Often when we talk about the invisible role that microbes have played in human history, we’re talking about the destruction wrought by diseases.

But as we’ll talk about in the next video, the life of Saccharomyces cerevisiae has intertwined with our own in ways that are both delicious and mysterious. Every time you eat a piece of bread or drink a glass of beer, you are participating in what might be (depending on how strict you are about your definitions) one of the longest-running microbiology experiments in human history. While the earliest scientific studies of microbes usually go back only a few centuries when it comes to yeast, the microbe at the core of some of our favorite foods and drinks, well...those past few centuries barely scratch the surface.

But before we get to that history, let’s start with some of the basic biology. Yeast are a fungus, though unlike many in that kingdom, they don’t grow the branching hyphae that characterize organisms like mushrooms. Instead, yeast grow primarily through a process of asexual reproduction called “budding.”.

While there are many yeast species found among different fungal phyla, perhaps the most well-known is Saccharomyces cerevisiae, which you might see labelled as brewer’s yeast or baker’s yeast. Along with a few other yeasts, these various strains of S. cerevisiae are what we’ve come to rely on for our beer, wine, and bread--and all through the magic of fermentation. Yeast, like many organisms, rely on different metabolic pathways to break down sugar and create energy.

These processes usually rely on oxygen. But to deal with situations where oxygen may be less readily available or some of their key metabolic processes are shut down, yeast turn to alcoholic fermentation. Fermentation helps the yeast get the energy it needs, but the particular chemical path they follow also produces ethanol and carbon dioxide.

The yeast doesn’t need these byproducts—in fact, too much ethanol could be toxic for them. But what is waste to a yeast is treasure to us, and that shift in metabolic activity has been making our bread rise and our brains woozy for millennia. Molecular studies of pottery jars found in China found that they may have held some kind of fermented beverage as far back as 7000 BCE.

And archaeological evidence has turned up for leavened bread from 2000 BCE in Egypt and 1000 BCE in North Western China. It’s easy to see why our use of yeast has persisted for millennia. To quote one group of scientists, it helped us make food and drinks with a “enriched sensorial palate” and also a “euphoriant effect.” But the long-standing ubiquity of yeast also emphasizes how mysterious their origins are.

We don’t actually know how yeast became involved with our cooking. We just know that it was ancient, likely starting with wine, and would come to span continents, potentially spreading through trade. The results were food and drinks produced through a series of observations about how factors like temperature, time, and air might affect the final product.

To quote Samuel Johnson’s 18th century dictionary, barm--a type of yeast--is “the ferment put into drink to make it work, and into bread to lighten and swell it.” At the time they didn’t know what was actually in yeast or how it worked to shape those results. We just knew it was a thing that provided…something…somehow…. Moreover, the earliest glimpses of yeast under the microscope seemed to support the idea that yeast was just some sort of chemical, a fermenting agent, not a biological entity of its own.

When Antoni van Leeuwenhoek observed yeast in the 17th century, he thought they were just bits of globular particles. Against the new and incredible bits of life he was seeing for the first time through the microscope, yeast seemed useful but ultimately unliving. But over the next few centuries, as more scientists worked to try and understand how fermentation actually worked, they identified not just the chemical reactions that shaped the process, but that yeast was an actual living organism essential to fermentation.

Eventually, Louis Pasteur would put all the pieces together, publishing a paper titled “The Memoir of the Fermentation of Alcohol”, or something like that, but in French. It was published in 1857 and it established the interplay between chemistry and biology that allowed yeast to turn to alcoholic fermentation to survive. He would also later uncover another form of fermentation carried out by bacteria, which is called lactic acid fermentation.

Pasteur’s work didn’t just change our understanding of how bread was made, it would go on to inform our understanding of germ theory, and how the invisible world of microbes connects to the grim realities of disease. From this early history, yeast have gone on to become perhaps one of the most studied eukaryotic organisms in modern biological research. Saccharomyces cerevisiae in particular has had an illustrious research career.

In 1996, its genome was fully sequenced, making it the first eukaryotic organism to be genetically mapped out. Part of what makes yeast such great research organisms is that they are easy to grow and manipulate--both through environmental and genetic means--which has allowed us to use them to study everything from DNA replication to prion diseases. In the same way that cultures have refined the ways we use yeast in our food and drinks, scientists have refined the way we use yeast in our lab experiments.

They’ve even developed genetic engineering techniques that repurpose yeast to brew the ingredients for everything from perfumes to antimalarial drugs. When you consider just how far we’ve come with yeast, you might begin to wonder: did we domesticate them? After all, our early days of yeast use likely coincided with our early days of plant and animal domestication.

So in the same way that we’ve come to rely on domesticated animals for food and labor, maybe yeast are our microbial workhorses. The answer to that question is one scientists are still studying, in part because the history of yeast is so large and mysterious, and in part because domestication has a very specific definition in these conversations. One paper we found describes it as “human selection and breeding of wild species to obtain cultivated variants that thrive in man-made environments, but behave sub-optimally in nature,” now that’s quite a number of nuanced requirements to check off.

But with this definition in mind, beer yeast seems to show the most evidence for having been domesticated, which is likely connected to specific aspects of the brewing process that have limited the exposure of the domesticated yeast to wild yeast. But all yeasts considered, the genetic analysis of beer yeast strains show that this domestication is actually relatively recent—as in, it only happened a few centuries ago. While it did happen before we even knew that yeast was a living thing that we were domesticating, it still came long after we started using yeast to make beer, leaving much of the organism’s history still a mystery.

It is often tempting to think that we have a microbial world and a human world that intersect only in chemical reactions and biological connections. But yeast reminds us that microbes are more than just a scientific reality, they are a cultural one--even when we didn’t know they were around. Yeast have created our foods, shaped our traditions, and both bonded and divided us.

And how we use and change yeast will only continue to shape our lives. Their future, tied so much to our own, is every bit as grand and mysterious as their past has been. There is but one inevitable way to end all this, with poop.

Because yes, even microbes do it. Not all that we eat can be retained in our bodies, something must exit. So come for the funny videos of microbes pooping, stay for the extra lesson on how they do it without a butthole.

Ah, food, one of life’s great pleasures. Whether you’re an organism whose body can create its own food, or you make do with the consumption of other organisms, food sustains and connects life both large and small. But we’re not going to talk about food today.

We’re going to talk about what happens…after. No great meal comes without consequences. Even photosynthetic microbes produce oxygen they don’t need, a byproduct that is quite useful to many other organisms, including us.

And for organisms that have to digest their food to extract nutrients, well, there’s all sorts of stuff left behind when the process is done. In the world of the microcosmos, there are not only many varied tiny organisms. There are also many varied ways to produce even tinier poop.

And when we say “tinier,” we mean it. If you’re worried about being grossed out by this video, or you really want to make sure you’re not missing anything, just know that some of this pooping will require you to pay very close attention. Some of this will probably look similar to macroscopic bowel movements, and some will seem less so.

So let’s start with the more poop-like poop that comes from some of the multicellular members of the microcosmos, particularly the ones with digestive systems complete with specialized regions for digesting food and releasing solid waste. Our dragon-like friend the gastrotrich is doing the old scrunch-and-poop, probably after having finished a nice meal made up of bacteria or protozoa. The food came in through its pharynx and was then sent through the intestine, a straight tube full of digestive cells.

After, the remains continued their way to the end of the tube, where lies the anus--a microscopic gateway out of the gastrotrich and into the world. That’s all well and good for the more complex organisms, but how does one poop when one does not have a butthole? Well, for some single-celled organisms, one becomes the butthole.

Such is the case for amoebas. Their formlessness allows them to easily shift their shape around to take in food via phagocytosis. The amoeba here happens to have found a nice meal made of a ciliate, which is now contained in a food vacuole filling with digestive enzymes.

Those enzymes will eventually break the ciliate down, allowing the amoeba to absorb nutrients into its cytoplasm. But then what happens with the remains? Well, as we see with this amoeba here, it gets released.

Amoebas and other similar organisms use a process called exocytosis to send waste back to the membrane where it is removed from their body. You can see it again here in this Mastigamoeba, slowly gliding along the microscope slide while a round bit seems to come off up top. It almost looks like it’s leaving behind a little piece of itself, which I mean, maybe it is.

Ciliates have a more refined approach to pooping, which is to say they have the unicellular equivalent of an anus called the cytoproct. Ciliates gather food into an oral groove, and then consume it in the digestive vacuoles that travel through the cell. Eventually, when the vacuole makes its way to the cytoproct, the contents get out, and there you have ciliate poop.

The way organisms poop—the way microbes poop—are even more varied and wonderful than we’ve been able to show you here. But you might be wondering how you even get from something relatively simple like this ciliate releasing a small bit of waste to something more complex, like this tardigrade with its more compartmentalized approach to defecation. Well the answer, of course, is evolution, but it’s an answer that leads to many more unanswered questions.

Even within animals, the evolution of the anus—or in some species, the cloaca—is not well understood, as discussed in a 2015 review paper with the spectacular title, “Getting to the bottom of anal evolution”. While the anus itself is linked to the evolution of a digestive system, not all metazoans have a designated anus. The hydra gut has only one opening, which means that the organism’s mouth has to pull digestive double duty and open back up to release the waste.

And how that system fits in with the evolution of digestion overall is unclear. Much like the evolution of sex, anuses are a trait that appears and disappears in evolutionary lineages, reflecting the fact that they might be useful for some animals and less so for others. The duality of the hydra’s mouth seems to be working out great for them after all.

But having a digestive system with a distinct entrance and exit is pretty handy too. For one, it’s just a little less disgusting. But it also keeps food flowing in one direction.

And that means animals like the tardigrade and us don’t have to wait to finish digesting to eat more food. That would be annoying. We just get to keep eating, all thanks to that evolutionary mystery that is the anus.

In the end though, anus or not, everybody poops. Food, after all, might be one of life’s great pleasures, but all good things must come to an end. And perhaps, with the microcosmos, we can find some pleasure in that as well.

If you need to nominate an ambassador to represent the microcosmos, you have to go with the tardigrade. They are weird, adorable, and very hardy,– a combination of traits that has made them many people’s first entry point into the microscopic world. And no matter how much time you spend with them, tardigrades will find new ways to surprise you.

In fact, when we’ve looked back on our journey through the microcosmos and our encounters with tardigrades, we’ve noticed that many of those strange sightings involve something that doesn’t immediately come to mind with tardigrades: sex. So today, we want to thread together these tales of tardigrade mating, and not just because it’s fun, but also because there’s a lot to learn from understanding how tardigrades make more of themselves. Let’s start with one of our early videos about tardigrades, where we were introduced a pregnant water bear whose eggs were beginning to develop even as her body remained still, and whose fate we’ll get to see play out later in this compilation.

We don’t know about you, but here on the Microcosmos team, we find it hard not to smile as we watch tardigrades with their cute, pitch-black eyes and little feet ambling across the smooth glass surface of the microscope slide. Moss piglets, water bears—whatever you call them, they’re fascinating to watch, and even more fascinating to try and understand. They are a puzzle, and we humans are the puzzler.

And so we figured, sure, we’ve already focused on tardigrades in a previous episode. But we cannot possibly leave it there. After all, like the microscopic world around them, tardigrades always have more to show us.

Our water bears are collected from ponds and growing moss, environments that give them the water and food they need to thrive. And so, behind the scenes, we like to give them a bit of attention, making sure that they are well fed and comfortable in their slides. After all, a drop of water is barely anything to us, but to a tardigrade, a thing so tiny and so good at surviving, that drop can be everything.

Tardigrades of course live their lives, absent of any kind of intention to teach us anything. They don’t even know we’re here. But watching is a form of interaction, and one that reveals the interface between the micro, the macro, and even the cosmic.

So let’s start with a part of the tardigrade body whose weirdness we haven’t had the chance to fully appreciate here yet: their mouths. The opening of the tardigrade mouth is a distinct ring, giving way to a tube that connects to the pharyngeal bulb, which helps the tardigrade suction out the juicy contents of its meal. But to get to the juice, the tardigrade first needs to pierce its target, relying on a pair of stylets that poke out of the mouth ring when it comes time to feed.

When the tardigrade molts, they shed their stylets and replace them with a new pair. Tardigrades are omnivorous, they’re open to eating bacteria, plants, and animals. For some species, their meal might even be another tardigrade.

But for the species here, our samples contain plenty of algae and plant matter for them to dine on, collected from the same watery and earthy worlds as the tardigrades these bits of organic matter keep them well fed like a built-in buffet. But there are other times where bearing witness to the microcosmos takes a little more adaptation on our part. Recently, we found a water bear with its half-shed cuticle filled with eggs, still attached to its body in what amounts to parenting in the microcosmos.

As you might imagine, we were more than a little curious to watch these eggs develop. But if you keep a slide on a microscope for too long, the water will evaporate, taking with it the moisture the tardigrade needs to exist outside of its dessicated, dormant state. So we kept these slides in humidity chambers and took extra care to make sure the coverslip wouldn’t crush the developing tardigrades.

And then…we watched. The tardigrade though was still. This video is from our sixth day of observation, the only sign of life being the absence of decomposition.

Three days later, we can see the eggs are developing, and there are even little tardigrade heads appearing. The mother still seems the same, attached to her eggs and seemingly still alive despite her complete stillness. When it comes time to hatch, the embryos will use their stylets and pharyngeal pump to make their way out, sucking in water to increase the pressure on the egg from the inside until the shell breaks.

But…we’re still waiting for that day to come with this tardigrade brood. Embryonic development in tardigrade can last from 5 days to more than 100, varying by species and dependent on environmental factors like temperature. The thing with biology, no matter what scale of creature you’re looking at, is that you’re always on their schedule, not the other way around.

Food and birth probably seem a bit domestic compared to some of the more recent tardigradian exploits you may have heard about. Earlier in 2019, Israel sent a spacecraft called Beresheet to the moon, taking with it, among many things, a digital archive of almost the entirety of English Wikipedia and also thousands of tardigrades, dried out in their ball-like tun form and ready for lift off. Seconds before Beresheet was supposed to land, however, it lost contact with Mission control and crashed instead, leaving one very obvious question: what will happen to those thousands of tardigrades, potentially spilled across the surface of the moon.

Our seeming penchant for sending tardigrades into space has taught us that they’re able to tolerate vacuums. Without liquid water, of course, the tardigrades will remain in their dormant state. Maybe if someone is able to return to that crash site decades from now, they might manage to find tardigrades that survived that crash, and there’s a chance that with a little water, they could come back to life.

We’re still working to understand all of the tricks of tardigrade survival, how does it play out in their DNA and proteins, and why some tardigrade species seem to be better than others. This is part of the fun of tardigrades and our collective fascination with them. We’re all learning together here on planet earth, like a giant science classroom, asking important questions like, “Is that round thing the mouth?” and “What does happen when you put a tardigrade on the moon?” Maybe this is our 21st century remix of Apollo, a conversation between microcosmos and good old-fashioned cosmos.

Science is ultimately a mix of deliberate and accidental questions, taking on many forms to suit our available tools. Sometimes science is a detailed study, and sometimes it’s a crash landing. It can be a collector of microbes and a master of microscopes, carefully tending to samples to meet the needs of their invisible residents.

And sometimes it’s you, watching along, making your own observations, maybe even thinking of new questions that no one has ever thought to ask before. In our next video, we’re jumping straight to the point. This is a video about tardigrade sex, featuring, you know, tardigrades having sex.

And the process is about as perplexing to watch as you might imagine tardigrade mating would be. Welcome to season two of Journey to the Microcosmos, and we are starting things off with a pun very much intended--a bang. Or more accurately, a multitude of bangs.

And we should warn you, it gets weird: there's poop! There's molted skin! There's the inexhaustible spirit of the tardigrade in flagrante!

So, how did we get here? Well, sex, obviously, if we're talking in the grand biological scheme of things. It is for some, an evolutionarily advantageous way of making offspring.

But we're talking about tardigrade sex right now in part because of a pregnant water bear that we started tracking in an episode last season. Even though the mother-to-be was completely motionless, we were able to see the heads of baby tardigrades forming in the eggs, and we were excited to record and share the moment of all those moss piglets hatching. Unfortunately, nature had different plans, and we'll talk about that more in our next episode.

But James, our master of microbes, has been hard at work documenting the lives of the tardigrades in his care. And that means, not only have we been able to find more tardigrades hatching, we've seen some particularly vivid demonstrations of the steps that came before. These hardy animals can reproduce asexually via parthenogenesis, but our recent samples have been full of frisky water bears, and that is what we are going to focus on today.

Sure, tardigrades contain an immense capacity for survival that astounds and boggles the mind, and it is a joy and a privilege to share the biology and visual wonders of their lives alongside all of the other tiny organisms we talk about. But sometimes, what the job asks for, maybe even demands, is that we film nature as it occurs. This is not what James expected when he ventured out to a river to collect some mud, even when it turned out that he had hit a microbial jackpot and counted at least 15 tardigrades on the first slide he prepared from the sample.

But as he continued to watch, he saw these two smaller tardigrades, both mysteriously drawn to the underbelly of a larger one. They seemed to be trying to reach into the belly with their piercing mouths, but the why's and the how's and the who's were all unclear. Their goal was mysterious.

And besides, tardigrades rarely demonstrate such a focused interest on much of anything. But twenty minutes later, those two jabbers were still at it. Their determination became much more understandable when the eggs inside the bigger tardigrade become visible, marking her as a female holding a number of unfertilized eggs, and marking the smaller tardigrades as males vying to do the fertilization in a microscopic love triangle (or as we call it at the Microcosmos headquarters, a "ménage à trois-digrade").

Was that too much? Who knows? So just how focused are these male tardigrades?

Well, remember that time we watched a solitary tardigrade poop? What a nice, surprisingly cute and relaxing clip that was. Well, now, let's watch a large female tardigrade defecate with apparently no care at all for her suitors, and watch those two continue poking the water bear anyway, undaunted by the cloud of poop around them.

But even tardigrades surrender eventually, and after thirty minutes of this poking and prodding, one of the males made a graceful exit, perhaps having lost out on the mating battle. With the path clear, the remaining male got a good hold onto the female's cuticle, sticking with her for a whole hour in their joint endeavor to fertilize her eggs. This couple was not the only one we observed in action.

In fact, this sample was full of tardigrades mating. And the behavior we observed was consistent, which is important scientifically. Given the collective fascination with tardigrades, it might surprise you to learn that their sex lives have not been well-documented.

Perhaps the most thorough study we were able to find is a 2016 paper that was conducted by scientists at the Senckenberg Museum of Natural History in Görlitz, Germany. It was published in the Zoological Journal of the Linnean Society, and their work chronicles the mating behavior of Isohypsibius dastychi, a different species from the one we are observing here. But their work was still a helpful basis from which to understand the various oddities and necessities of water bear sex.

The adulthood of tardigrades is often defined by the number of molts it takes for them to reach sexual maturity. For example, the female I. dastychi develops eggs during her third molt. But at this stage, the female doesn't shed off their molted outside, called the exuvia.

They molt their cuticle...but then they stay in it...like wearing your shed skin like a coat, and why not? They stay in there, along with their eggs. When male tardigrades were added to the mix, they made a beeline for these egg-bearing females.

In the exact technical terms used by the scientists in the paper, the process starts with the male "bringing his cloaca close the mouth opening in the exuvia." And thus begins a process of "mutual stimulation," the female uses her stylets and sucking pharynx to prod at the male's abdomen as part of a mating process that will take, similar to our tardigrades, about an hour. It all ends, of course, with ejaculation, but how that fertilization occurs can vary. In some species, sperm might enter through an opening in the exuvia.

In others, the male might use their stylets to poke holes in the shed cuticle that they then deposit their sperm into. One species has been seen using both of those methods. The observations that have been documented are that challenging combination of rare and different, where it's not clear how everything fits together.

And in cases like the species we're looking at right now, that means the mechanisms actually remain unclear. The sexual habits of this species are undocumented, this is science, right here, you’re watching it. As for when and how the sperm is deposited, either we weren’t able to film the exact right moment, or it happened so subtly that we missed it.

In short, it’s different for different species, and it is not well studied and even though we had a slide full of frisky tardigrades, it was still a challenge to observe. Sometimes, here at Journey to the Microcosmos, we end our episodes talking about how observing the microcosmos gives us new and better perspectives on the whole rest of the world. But sometimes, we’re just excited to have the opportunity to see something so few people have seen, and that’s just interesting… because it is.

And hopefully this video will be part of a hopefully ongoing tradition to record tardigrade mating. And next week, we’re going to be back with the fruits of their labor, to show you the highs and the lows of tardigrade birth. We’re going to be returning to the fates of the tardigrades in those first two videos in a bit.

But before we do that, we wanted to digress a little. We ended that last video with the hope that we would be part of a developing tradition of recorded tardigrade mating, a necessary step in developing our understanding of how tardigrades reproduce. Well, a few years later, we returned to the tardigrade to learn more about a much more established tradition— that is the tradition of people coming up with strange theories about tardigrade sperm.

This is not an organism. At least, it’s not yet anyway. What we are looking at are a lot of zoospores packed together, getting ready to be released so that they can find the right place to grow into themselves.

And this? Well, this is a male tardigrade filled from head to many toes with sperm. Just chock full of it.

We did not start this channel with the intent of becoming so familiar with tardigrade mating habits, but tardigrades are easy to find, and their mating habits are straightforward enough to track, so we keep getting drawn back into the details and the drama of tardigrade reproduction. In the past, we’ve talked about the particulars of their mating habits. And today, we’re going to use this tardigrade as a convenient starting point to talk about the history of our understanding of sperm.

Because as absurd as it might seem to be looking at tardigrade sperm, the theories people have had about sperm are much weirder. Humans have been theorizing about the contents of seminal fluid long before the invention of the microscope. We did not, as a species, need to work out all of the scientific mechanisms underlying sexual reproduction to make babies.

But the observation of children who carried traits from their parents naturally raised questions about how those traits were passed on. And those questions inevitably led to some hypotheses. And look, not all hypotheses need to be good.

In fact, many of them are terrible. It’s just that when you’re trying to understand the world, you need to start somewhere, and as long as you’re willing to work with your ideas and challenge them, then hopefully some day, you go from somewhere, to somewhere better. Some of these early ideas about sperm weren’t just bad though, they were, maybe as we should expect, a little self-serving to the male ego.

Masturbatory, even, you could say. Like the theory now known as spermism, advocated for by one Pythagorus, yes, that Pythagorus. According to spermism, the information driving heredity came entirely through the semen.

Whether that was the color of someone’s hair or their height or whatever other trait— all of that came through the father, and the father’s father, and so on. The mother’s role was just to provide nourishment to the sperm as it developed and manifested those traits. It’s easy enough to find flaws with this idea.

After all, our observed reality, which Pythagorus just chose to ignore, shows us that traits pass from mother and father alike. But this would not be the last theory to overestimate the sperm. Even with the advance of scientific tools over the ages, there was plenty to get wrong.

One of those key scientific tools was, of course, the microscope, which allowed scientists to observe the cells within semen for the first time. But just because microscopists could see sperm now, that did not mean they could make out the details. And this invited speculation, including the idea that we are essentially developed from a much smaller version of ourselves.

This theory was called preformationism. At the end of the 17th century, the Dutch scientist Nicolaas Hartsoeker sketched out his spermist vision of preformationism: inside the head of the sperm cell, he speculated, was a very, very, very small human-- later called a homunculus-- just waiting for a womb to grow in. And look, we have the advantage of centuries of advancements to know that the homunculus was wrong.

But maybe take a moment to think about how weird that would be if it were right. Look at this tardigrade, full of sperm. And then just imagine if we zoomed in further on those sperm cells, and we could see little tiny tardigrades inside of them.

And then, if we could zoom in even more on those tinier tardigrades, we’d see even more tinier tardigrades inside of them, and so on and so on. Now, to be clear, that is not what tardigrade sperm looks like. Their sperm is made up of a slightly globular head connected to a flagellum.

But as wrong as both spermism and tiny people inside of sperm turned out to be, they’re not that far off the mark if you think about the zoospores we showed at the beginning of the episode. They are a tool of asexual reproduction in a lot of different organisms, including algae, fungi, and bacteria. They all use zoospores in different ways.

And because we don’t know exactly what organism these zoospores come from, there are specific details of their lives we just don’t know. But take, for example, the Phytophthora, a group of oomycetes most notorious for their role in the 19th century potato blight that led to the Great Famine in Ireland. Phytophthora can reproduce sexually, but they mostly rely on making asexual zoospores, which pack together in a biological enclosure called the sporangia until the conditions are right for their release.

And when they emerge from the sporangia, the zoospores start swimming, using their flagella and chemical sensors to find the ideal place to settle and germinate. All of the hereditary information in those zoospores came from one parent, fashioning an organism that resembles the infant form of that parent, and that seeks only nourishment from its new host. The outlines of that process sound pretty similar to how early philosophers and scientists envisioned their sperm-centric ideas of sexual reproduction.

But what those scientists missed— what separates the sperm from the zoospore— is the nature of sexual reproduction itself, of the genetic variation it drives by combining and recombining traits from different parents. Sperm cannot accomplish that on their own. Sperm needs an ovum to fertilize, which scientists would eventually uncover in the 19th century with experiments on rabbits, sea urchins, and starfish eggs that helped to further our understanding of the molecular and cellular collaboration that set the stage for inheritance.

Yes, my friends, as Napoleon drove to conquer Europe, we had no idea that sperm and eggs came together to make the next generation. There are a lot of different types of spores out there, used for both sexual and asexual reproduction. And in contrast, sperm might seem more narrow and focused, both in diversity and purpose.

But the function of sperm is so specialized that it requires its own diversity. Even among tardigrades, there are different morphologies to the head and middle structures of the sperm, small differences that distinguish between species. And that’s because however weird our theories about sperm have been, the reality is also very strange.

Of all the cells in the body, sperm cells are maybe the only ones that are expected to navigate and survive in two different beings. The homunculus was wrong, but the idea of there being an image of our life passed on within sperm and in spores, it isn’t absurd at all. It’s just too literal.

Instead, that image is abstracted and tucked away into genetic information. And zoospores and sperm alike are just packets of that information, swimming off to new and different futures. So far we’ve seen the eggs, the sperm, and the very messy process of getting the two to meet.

And for our last video, we want to focus on what all this is meant to be for: baby tardigrades! We’ll also find out the fate of our pregnant water bear from the first episode, and see what lessons we learned in the process. Hello, everyone, and welcome back to the second half of our tardigrade reproduction spectacular.

In our last episode, we got a very close look, though I suppose all of our looks are pretty close, at the various biological intimacies exchanged by tardigrades as they mate. Multiple males contended for the heart, or at least the unfertilized eggs, of a female tardigrade by poking endlessly at her exuvia. It was really quite dramatic, and if you haven't watched it yet, you should.

It’s great. There’s poop. But now, it is time for the thrilling conclusion to all of that mating: the babies.

This tardigrade has been lugging her future offspring in a sack made up of her exuvia, which is her shedded exoskeleton. Like a spare room that's been converted into a baby nursery, the same exuvia that started as the mother's own cuticle before it became the site of courtship, has now undergone another renovation to become a handy, portable egg nursery. The ways that tardigrades deal with their eggs may look different for different species.

While this water bear carries her exuvia at the end of her body, some species carry it more towards the front. Some don't lay eggs in exuvia at all, while another species has been seen depositing their eggs in the exuvia of water fleas. Why our species has evolved or chosen this method is unknown to us.

Maybe it allows her to provide them protection, or perhaps she can ensure they hatch in the best possible location this way. We just don't know. But we did very much want to observe this mother-to-be.

And so for three days, we followed her, doing our best to also take good care of her. Now, the biggest concern was making sure she wouldn't get too dry, so we had to keep her away from the edges of the slide. To do this we used only the most advanced techniques: we poked her with the fine hair from a brush to push her towards the more watery regions of the slide.

We even set an alarm to remind us to add a drop of water to the slide every four hours. Yes, even at night. That’s a lot of alarms.

Now, you might be thinking--aren't tardigrades supposed to be the hardiest creatures on Earth? Able to withstand extreme conditions, hypothetical apocalypses, and maybe even crash landings on the moon? Surely they do not require this level of helicopter parenting (or I guess it’s grandparenting?).

Well, we do care a lot for all of our microscopic organisms, and so it could be that this watery vigil is just an abundance of caution on our part. But we've seen with our own eyes that no matter how tough tardigrades may be, the microcosmos is tougher. Sometimes things just...happen, and not in the way you might hope.

You might remember this tardigrade from a previous episode. We were pretty excited about her because we could see all of those eggs just ready to be hatched. But the mother was completely still.

She hadn't moved at all, which would usually be worrisome. But she wasn't decomposing, so we thought maybe she was still alive, just biding the time until her eggs hatched. And so while she waited, so did we.

Gestation in tardigrades can vary quite a bit, not just by species, but with temperature and just plain old chance. It generally takes a few days, but it can take months. We kept her in a humidity chamber and checked in to make sure the coverslip wasn't crushing her.

But the inactive tardigrade never became active again. Eventually, her body did begin to decompose, as did the eggs inside her. And so, this tardigrade family was not meant to be.

We don't know what killed this tardigrade, but it's hard not to feel responsible for the organisms we film. Sure, they're unaware of our existence, but they are in our care, and like the animals and humans we come to feel connected to, it's sad to be reminded that endings can come for even the mightiest creatures among us, even if they're also among the smallest. That's why, when we later found the impending tardigrade mother that started this episode, we took so much care to keep her alive.

It was nerve-wracking because we didn't want to lose another brood, hence the 4 hour alarms and the brush hair. So you might be able to imagine our combined excitement and relief when after 4 days of this intensive care, we saw a tardigrade cuticle that was no longer full of eggs--it was full of egg shells and baby tardigrades! Now unfortunately, it turned out that getting footage of the actual hatching during this moment was challenging.

The eggs would roll around in the cuticle while also being dragged by the mother. But you can see the baby tardigrades clawing their way through the mass of eggs. Eventually though, the mother finally completely shed her cuticle.

You can see it here, it almost looks like a tardigrade ghost chasing behind her. This moment was particularly fortuitous for us because as the tardigrade rid herself of the exuvia that had housed her future children, one unhatched egg came out as well. It's weird to look at this and realize that in a few moments, out will come a tardigrade.

Just an oval with some lines in it, a formed thing packed away. That arrow-shaped thing that's pumping away is the tardigrade's stylets, a part of their mouth that pokes out when it comes time to eat and it helps the tardigrade suction out the meatiness of the microbes that it feeds on. But before they can get to eating, the stylets have an important task: helping the baby tardigrade get out of their shell.

After two hours of struggling, the stylets of the tardigrade begin to move more repetitively until finally, it's able to poke holes in the egg. These holes allow water into the egg, increasing the pressure inside until the egg just pops and out comes a little moss piglet. And with it, a new generation is born, adding to a lineage of mighty microscopic animals stretching back hundreds of millions of years. *sigh* If you heard a sigh just then, it's a sigh of relief and joy from all of us at the Microcosmos team.

When we set out to track and record a tardigrade birth, we didn't know that it would lead to both heartbreak and excitement. But it's all worth it for this little buddy among many. We hope to see them all again, perhaps creating their own brood.

But for now, paddle away, our little friend, the whole microcosmos awaits. Hi, I’m Sam Schultz. I’m new here on Journey to the Microcosmos and I’ve been put up to a very questionable task for my first episode.

While the other new hosts get to talk about beautiful microbes like the Beggiatoa, I am here to talk to you about butts. A whole compilation of them. And I know why I’ve been asked to do this.

I’m a producer for SciShow and one of the hosts for the lightly competitive science game show podcast SciShow Tangents. And each of our episodes ends with a butt fact, where one of our panelists recites a fact about butts or poop or something vaguely butt-related. We’ve also been known to argue about important existential questions that can shake you to your very core, like…is butt legs?

But as someone who is on Team “Butt is Legs,” I have to admit that microbes make a compelling argument for the opposition, because so many of them don’t have legs but still have a butt. Like in this first video, about the flowery-looking animal called Bryozoa. When you first see them, you’ll probably be like, “Wait, where’s the butt?” But stick around, and you’ll find out why that’s actually a very important question for scientists studying the Bryozoa.

If I were to ask you to just do a little sketch of what you think of when you hear the phrase “moss animals,” you might imagine something like this tardigrade—a waddling, slightly fantastical creature that we did in fact find living within the minuscule greenery of moss. But depending on who you are and what you know, maybe you draw something completely different. Maybe you draw something more like this.

These don’t really look much like an animal at all, do they? At first glance, they seem a bit more like plants, with a stalk connecting a series of flowers with thin, elegant petals. But no, that is indeed an animal.

One that has the dubious honor of being defined largely by its anus. Before we get there though, let’s start with just explaining what these animals actually are. These organisms are members of the phylum Bryozoa, a name that translates in Greek to “moss animals”.

But Bryozoa aren’t actually animals that live in moss. They get their name because of what they look like when all of the elegant individuals you see here congregate into colonies that look, ya know, mossy. That giant gelatinous mass seems to bear little resemblance to the tiny winsome creatures that make it up.

But Bryozoa are notable in the animal kingdom because of just how colonial the phyla is. Around the world, in waters salty and not, are colonies of different shapes and sizes made up of individuals like these. These colonies can get their start in different ways.

Some might be the result of sexual reproduction, sperm and egg that could have met in the waters away from their makers. Or, in other cases, one colony may have captured sperm released by another, using it to fertilize its own eggs, which stay brooding until a larva emerges, ready to set out and develop its own colony. But Bryozoa have another asexual option for reproduction.

Instead of fertilizing an egg, a colony can amass a bunch of cells into a structure like this one, called a statoblast. These statoblasts can be so prevalent in our samples that James, our master of microscopes, has to scoop them out to keep them from being the main focus of our videos. They are thick and durable, able to lay dormant and protect their contents from cold temperatures and dry weather.

But when the conditions become right again, they grow into a new individual that will form the basis of a new colony. To do that, the individual Bryozoa, called a zooid, navigates the waters as a larva, eventually attaching itself to some kind of solid surface. Perhaps a piece of rock, or some kind of shelf, maybe even some seaweed.

And from there, it will begin to divide, budding off but keeping its descendants close and connected as they increase in number. Now, the arrangement of these zooids dictates the shape of their colony, as does the material they secrete to support the colony overall. Some excrete a gelatinous material that turns them into those masses of jelly we saw earlier.

Others rely on chitinous materials that give them more rigidity. And as they spread over their surface and even grow up into the water around them, Bryozoa colonies take on many spectacular shapes. Even though Bryozoa are different from coral, they definitely look similar.

And like coral, Bryozoa colonies don’t move. Instead they remain attached to their surface. But while they may not be able to move through the world around them to gather resources, Bryozoa are able to make the most of their surroundings, with individual zooids taking on special functions that allow the group to accomplish more than the individuals.

Some focus on making eggs, while others form small beaks that allow them to snap at predators. And these zooids are all dependent on another group of zooids that gather food and share the nutrients with everyone else. To get that food, the feeding zooids rely on the lophophore, the ring of tentacles crowning every zooid’s bodies that looks a bit like a bunch of thin petals or a very elegant duster.

Those tentacles are covered in cilia, which circulate the waters around the Bryozoa, stirring currents like the ones you see here so that it can sweep bits of food like plankton towards the animal’s mouth. And from certain angles, you can see how the lophophore and the rest of the zooid’s organs tuck into a sort of casing, which is a structure called the cystid. And while the Bryozoa has various other parts that we could talk about, what we’re really here for is the anus, which is a much bigger deal than you might expect from an animal with such a simple digestive system.

There’s a mouth, an esophagus, and then a u-shaped gut that leads to the anus. Now the issue is really rooted in the existential crisis at the heart of microcosmos: how are things related to each other? And what does that mean for the names that we give to things?

Organisms like these were originally given the name Polyzoa in 1830, but that was changed a year later to Bryozoa by the German naturalist Christian Ehrenberg. But as more organisms were added to the group, the people watching them and describing them realized that they were actually lumping together two very similar groups of organisms that actually had some key differences between them. And one of those key differences was where the organism’s anus was located.

One group had their anus situated just outside their lophophore—that tentacle crown we were watching gather food earlier. And one had their anus situated just inside their lophophore. So to differentiate between the two groups, scientists gave them very appropriate names.

One group was called Ectoprocta, which translates in Ancient Greek to “outside anus”. And the other were called Entoprocta, which means “inside anus.” Because you can define anything by its butt, as long as you turn to ancient languages. In general, Bryozoa typically refers to the Ectoprocts, though for many reasons (..

I can’t possibly imagine why…), Bryozoa seems to be the preferred name. But the history and relationship between the Ectoprocts and Entoprocts remains an ongoing mystery that scientists continue to explore. They are not the only creature out there to be wrapped up in a phylogenetic mystery.

But isn’t it great to know, especially for such a strange animal like this, that the mystery is built on butts? Imagine if your name translated to “inside anus” or “outside anus”? I guess it sounds better in Ancient Greek, but still, scientists really make a lot of strange choices.

Like the subject of our next video: Antoni Von Leeuwenhoek, the original master of microscopes. Leeuwenhoek looked for microbes in all sorts of interesting places. But given the subject of our video today, I think you can figure out one of the more unique sources of microbial delights that Leeuwenhoek found.

In the year 1675, I discovered very small living creatures in rain water, which had stood but few days in a new earthen pot glazed blue within. Those are the famous words that Antony Van Leeuwenhoek used to describe microbes. Though he didn’t use the word “microbe”, he called them animalcules, and the letter itself is not exactly the one he wrote.

Leeuwenhoek’s version was written, of course, in Dutch and then sent off to Henry Oldenburg, the secretary of the Royal Society in London who founded the publication Philosophical Transactions. Oldenberg translated Leeuwenhoek’s work into English and published it in 1677. The rest, as they say, is microbiology.

In 1932, the scientist Clifford Dobell published a biography of Leeuwenhoek called “Antony van Leeuwenhoek and his “Little animals,”. Dobell introduces his subject with some self-consciousness for taking on the story of someone who, if you’ve been interested in microscopy, you have almost certainly heard of. And he wrote: Dear reader: I know full well that you and everyone else must have met Mr van Leeuwenhoek many a time before; but please let me reintroduce him to you, for he is a man worth knowing more intimately.

Though he was born exactly 300 years ago he is still very much alive, and would be glad to make your better acquaintance—provided only that you are a “true lover of learning” (as of course you are). The fascination with Leeunwenhoek’s work for Dobell and so many of us lies in the scope of Leeuwenhoek’s curiosity, and the writing that documented his many journeys into the microcosmos. Born in the Netherlands in 1632, Leeuwenhoek was a draper by trade, meaning that he sold cloth.

He took up microscopy as a hobby, learning to grind lenses and turn them into a tool to see into tiny worlds. The simple microscopes he built were better than many of those of his contemporaries, producing images that were both more magnified and more clear. Those of course compared to what the average high school biology class has today, they were very bad at both of those things.

The creatures Leeuwenhoek described in his 1677 publication were not his first description of tiny animals in water though. In 1674, he wrote of “green streaks, spirally wound,” a description that later observers have taken to refer to the filamentous green algae spirogyra. He also wrote of little creatures he’d found that were, as he described them, “a thousand times smaller than the smallest ones I have ever yet seen upon the rind of cheese.” Those little cheese rind creatures he was describing were likely mites.

Reading Leeuwenhoek’s work is kind of like following an old treasure map. His words lay out pathways and clues that help us navigate through a drop of 17th century water. And if you follow that map, at the end you find a wealth of information, the knowledge of microbes and their identities that was still unknown in his time, but that in the centuries since has been accumulated.

For Dobell, Leeuwenhoek’s 20th century scientist biographer, this treasure hunt was a long one, borne out of his own enchantment with these original discoveries and a desire to better understand them. He learned Dutch, only to find that the 17th century Dutch of Leeuwenhoek’s letters was impenetrable to him And with World War I raging around him, he had to set aside his attempts to translate Leeuwenhoek’s work for some time. But eventually, through what seems to be largely a lot of determination, Dobell was able to work his way through Leeuwenhoek’s letters.

And that labor of love led, naturally, to translating things like Leeuwenhoek’s descriptions of his own poop. Per Dobell, Leeuwenhoek wrote the following: I have usually of a morning a well-formed stool; but hitherto I have had sometimes a looseness of the bowels. […] My excrement being so thin, I was at divers times constrained to examine it. But Leeuwenhoek wasn’t documenting his stool just for the sake of it.

In that sample, he saw an opportunity to observe the animals within. I have at times seen very prettily moving animalcules […] Their bodies were somewhat longer than broad, and their belly, which was flattened, provided with several feet, with which they made such a movement through the clear medium and the globules that we might fancy we saw a pissabed running up against a wall. That description sounds promising, but Dobell had a problem.

Leeuwenhoek had compared his microbes to a “pissabed,” and like us, Dobell had no idea what a “pissabed” is. Even the original English translation he’d read shrugged it off as a Dutch term with no obvious English equivalent. Dobell had to find a 17th century Dutch-English dictionary to figure out that a “pissabed” is a woodlouse.

And with that reference point, Dobell had enough to diagnose Leeuwenhoek—very belatedly, of course—with Giardia, the product of a flagellate known as Giardia intestinalis. At the time that Leeuwenhoek was identifying the microbial residents of his stool, he almost certainly didn’t know their role in his poor digestion. Nor did he of course have any name for them, or any of the other microbes that we now know he saw.

And yet we can identify rotifers from his characterization of their wheeled heads, and vorticella from his description of their belled bodies. The irony of our centuries-later fascination with reconstructing and replicating Leeuwenhoek’s work is that his contemporaries had a much harder time of it. Leeuwenhoek didn’t help his cause much, refusing to provide extensive details of his methods and even keeping his microscope-building techniques to himself.

He did, however, provide eight signed testimonies from various men, including a minister, to verify that he had made these observations. Of course, signed testimony is one thing. Reproducibility is another.

Several of Leeuwenhoek’s contemporaries tried and failed to replicate his observations until finally Robert Hooke—the famed microscopist and writer-slash-illustrator of Micrographia—was able to see the tiny animalcules Leeuwenhoek had seen. Hooke however was willing to outline his specific methods and demonstrate them, validating the observations Leeuwenhoek had made and providing others with the means to make their own microbial ventures. And like Dobell centuries later, Hooke was reportedly so taken with Leeuwenhoek’s work that he learned Dutch to read his letters.

The legacy of Leeuwenhoek is in the microbes he uncovered, but of course it was more than that. It was also the curiosity that he passed on, and the acts of interpretation that they inspired—both literally with all of these scientists taking up Dutch to just learn from his letters, and then scientifically as we decipher what microbes he must have seen. And so it seems fitting to close with how Leeuwenhoek viewed his own little animalcules.

He said, “Among all the marvels that I have discovered in nature, these are the most marvelous of all.” I know I’m supposed to think it’s gross that Leeuwenhoek dug into his own poop for microbes. But as weird as it is, I also respect the dedication to his craft. But now I want to know how many other noted scientists have documented and studied their poop so thoroughly that future historians can diagnose them with an intestine parasite.

Is it just Leeuwenhoek, making him a true legend? Well, I’ve probably spent too much time already thinking about Leeuwenhoek’s butt. Which brings me to my next question.

What’s the big deal about butts anyway? Why do we really need them? Luckily there’s an animal that can teach us that: flatworms, a worm so simple that it doesn’t have an anus.

There are no ghosts in the microcosmos. There is no Halloween in the microcosmos. But if there were, this flatworm would fit in quite nicely with its spectral appearance.

It might be navigating its way through a field of other organisms, but it looks like it had three minutes to prepare for a costume party and decided to go with the classic “old bedsheet with eyeholes” ghost costume. And maybe comparing this flatworm (or even worms in general) to a ghost is kind of apt. Ghosts straddle two worlds, and so do worms, though in their case, instead of bridging the realm of the living and dead, worms manage to stretch between the micro and the macro.

This creature you’re watching now would probably be difficult to see without a microscope. But on the other end, there are worms that are meters too long to observe with a microscope. And akin to ghosts, worms might seem like a sort of reduced form of life.

After all, for a lot of us, worms are the wiggly tubes that we dug up from playgrounds. And of all of the worms, flatworms are perhaps the most simple of all. But even digging into what it means to be the simplest worm makes the whole notion of simplicity seem ludicrous.

So let’s start with the broader question: what are worms, other than wiggly, self-mating tubes? While we generally use the word “worm” to describe many different, unrelated, invertebrate, tubular animals that belong to a number of different phyla, there are three main phyla that people focus on. The first are our flatworm buddies here, who are known more formally as Platyhelminthes.

Then there is the phylum Nematoda, known less formally as roundworms. We’ve come across nematodes many times in our journey through the microcosmos, which makes sense given that they are one of the most abundant animals on this planet. And last are the Annelida, phylum of our playground friend, the earthworm.

While earthworms are a bit beyond the scope of the micro, if you will, we’ve also got this little Stylaria lacustris. At the tip of its head is the proboscis, an antenna-like structure that the worm uses for feeding. And as we scan past the head, you can see that the length of the Stylaria’s body is segmented.

Of the three phyla we’ve mentioned, annelids are the most complex. They have a closed circulatory system with tubes that transport nutrients and oxygen throughout their body, as well as a complete digestive system that has both a mouth to take in food and an anus to get rid of waste. If we think of worms in general as a gut enclosed within a body wall, then that gut is a tube and the body is a tube, and you can imagine that worms are a tube inside a tube.

Now what separates these different groups is the space that separates one of those tubes from the other. Complex systems inside annelids are due to what’s called a coelom, a tissue-lined cavity that sits between the tube of the digestive tract and the tube of the body wall. Importantly, this cavity is filled with fluid, which facilitates the development of organs along with the transport of nutrients around the body.

The development of the first coelom was a very big deal because coeloms don’t just allow annelids to have complex systems. They connect the seemingly simple organisms we’re talking about here to more complex animals—like us! Because annelids and humans are both what is known as coelomates, or eucoelomates.

We are animals that have coeloms. So, thank you to the ancestor that we share with earthworms who developed the first coelom, who allowed more complex organisms like you to exist. Nematodes, on the other hand, are what are called pseudocoelomates.

They still have a coelom-like fluid-filled cavity, but the cavity isn’t lined with tissue like you find in true coeloms. Other microscopic pseudocoelomates include gastrotrichs and rotifers. This pseudocoelom is a slight but significant difference in nematodes when compared to annelids, corresponding to a slightly less complex body plan, though nematodes still do have a complete digestive system.

The flatworms are the simplest of all. Starting with the coelom: flatworms don’t actually have one. They are considered acoelomates.

That area between the gut and body wall where annelids and nematodes have a cavity full of fluid is instead full of tissue. This has a few consequences for the flatworm, but one of the biggest ones is that this area can’t support the development of specialized systems that you see in other animals. The lack of coelom imposes limits on the flatworm’s body and lifestyle, reducing both its complexity and size.

But that does not mean the flatworm lives a simple life. Of the three worm groups, flatworms are the most like a tube within another tube, except that their outer tube is much less cylindrical. The “flat” in “flatworm” is actually an adaptation: without a circulatory system or respiratory system, the worm relies on diffusion across its outer membrane to supply cells with oxygen.

Being flatter gets the cells closer to the outside and better positioned for diffusion. And without the ability to build a complete digestive system like those found in nematodes and annelids, flatworms have to adjust to life without that most under-appreciated body part of all: the anus. They take in food through one opening, digest it in their gut, and then spew the waste back out from that same opening.

So unlike other organisms who have separate entrances and exits for their food, the flatworm must wait to finish digesting before it can eat again. But this is not the only way flatworms can get rid of waste. Their body is lined with a special type of cell called a flame cell that also gets waste out of the worm.

While it may not have the elegance of an interconnected set of organs, this system is its own form of complexity. Basically, every flame cell is one single-celled kidney, allowing the worm to just... ooze waste from its skin. And this is just scratching the surface of the biological intricacy flatworms are capable of.

Like many of their more involved worm counterparts, flatworms can reproduce both asexually and sexually. And when reproducing sexually, they can mate with another flatworm, or even with themselves--both making and fertilizing their own eggs. There’s also those flatworm species that can regenerate, a trick that our more developed bodies are, alas, not capable of.

Even the many weird ways they move make the word “wiggle” feel reductive. Our perception of simplicity across nature is built on comparisons. Compared to us, an earthworm may seem simple.

Put that earthworm next to a flatworm, and suddenly the earthworm represents a gigantic biological advancement. But then put that flatworm next to a bacteria, and suddenly it contains multitudes, it is a universe. That flatworm is a marvel, an almost unbelievable testament to the power of evolution.

I never really thought about how hard it would be to eat if I had to poop out my mouth at the same time, but yeah, I guess I have a newfound appreciation for the fact that our bodies are equipped with an easy exit strategy for waste. But you know who I wish would poop a little less? The subject of our next video.

Because now I can’t help but look around my house and wonder how much of it is covered in their poop. One of the most frequent questions we get asked on Journey to the Microcosmos is how James, our master of microscopes, stays safe while gathering and studying samples. After all, as beautiful as the microcosmos is, there are plenty of parasites and pathogens in there that we’d prefer to keep under the microscope and out of our bodies.

But while James has simple and effective guidelines to protect his health—like making sure he always washes his hands before and after handling samples, and never venturing into a pond with an open wound—there are times when those measures don’t quite cut it. These are the times when no matter how much you protect yourself outside or inside, the microcosmos gets the best of you. These are the times of the dust mite.

Now we don’t usually start these videos with a warning, but we’re going to do that today. While working on this video, at least 4 members of the Journey to the Microcosmos team found themselves having a bit of a crisis about how clean their homes were. If you’ve found yourself sneezing and wheezing and running around miserable with a runny nose because of dust, this arachnid is probably the culprit—though perhaps not in the way you would expect.

Dust mites are small in the grand scheme of things, but large in the microcosmos, hitting just around 300 micrometers in length. Their bodies are decked out with four pairs of legs ending in feet that act like suction cups as they wander in their quest for three things: water, food, and darkness. Dust mites love humidity because their lives depend on it.

They don’t drink water so much as they osmosis it into them, relying on a salt-filled gland near their mouth to absorb water vapor from the air around them. So if their surroundings become more arid, an adult dust mite will wither away until it eventually dies. This is a challenge for dust mites that live in places with seasons, where wet and rainy falls give way to dry winters.

To survive those shifts, they rely on the resilience of their younger brethren—the nymphs—whose forms are able to withstand the lack of moisture. The seasons of the dust mite’s life was one of the important clues that helped scientists in the 20th century understand their role in dust allergies. Scientists identified dust as the source of some kind of allergen in the 1920s, but they were not sure what it was exactly about dust that did it.

They just knew that people all over the globe suffered from dust allergies, and that these allergies were often seasonal, peaking in the autumn, particularly after warm, humid summers. And other possible sources, like animal dander or mold, just didn’t quite fit right with the seasonality of the allergy. But in the 1960s, scientists in the Netherlands and Japan realized that the dust mite might be the culprit.

They’re found in large numbers in the dust that lines the unwashed and untouched corners of our homes, their populations peaking in time with the runny noses and bleary eyes of dust allergies. But while experiments confirmed that mites really were the source of people’s sensitivities to house dust, it wasn’t clear what made them so special. So, we’ll get to that in a bit...

James often finds dust mites in the humidity chambers he uses to keep his microbes alive. These chambers are made up of a dish lined with wet toilet paper, which is just about all a dust mite needs to feel right at home—especially when there’s potentially some delicious microbes to munch on. So to keep those other microbes from becoming a mite meal, James has to regularly clean out and disinfect the humidity chamber.

Now, that cleaning might get rid of the dust mites in the humidity chamber, but the problem is that our houses are full of food for them because our houses are full of us—of flakes from our skin that shed and gather all around and sustain the invisible mite. And this is a problem not just because it means the mite can thrive in our homes. It’s a problem because food means poop.

Over the course of its life, a mite will produce about 1000 pieces of poop that are roughly the size of a grain of pollen. Inside those bits of fecal matter are enzymes that help the dust mite eat its own poop and get nutrients that it might not have gotten the first time around. But should the dust mite choose to not revisit its prior meals, the feces will float around the room, attached to other particles until eventually they settle down—perhaps on a pillow, or on a pet’s bed, or a car seat.

It’s like we’re living in a gigantic snow globe, except that the snow... is dust mite feces. In 1981, researchers confirmed the bad news about this animal’s poop— people are allergic to it. To be more specific, they confirmed that dust mite poop contains specific proteins that many people are allergic to.

So if you’re airing out some sheets and you start sneezing, what you might actually be reacting to is dust mite feces flying around the air. Now, we apologize for this mental image but it is reality, and it has embedded in our heads, so we have to share it with you as well. But we can offer what might be a small comfort: there is another allergen from dust mites that has nothing to do with poop.

Dust mites have a fairly lengthy mating process, sometimes taking up to two days to finish—this is a pretty lengthy time for any organism, but especially for an animal that’s only got about 100 total days to live. Over the course of their life, the female dust mite will lay up to 80 eggs, which hatch into larvae that then go through several different stages of development before becoming adults With each passing stage, the dust mite sheds its exoskeleton, leaving behind its youth. And that exoskeleton provides some of the other allergens for people react to.

But though that is something people are sometimes allergic to, it really is mostly just the poop that sets off people’s allergies. So I guess there really wasn’t that much comfort there after all. And even for a trained and cautious microbiologist, dust mites can become an unwelcome surprise.

Once, James brought home some samples taken from a water dish that his neighbor left outside for their cat. The surface was covered in tiny round things, and James thought they might be rotifers. But when he looked at the surface of the scum under the microscope, there were no charming rotifers to be found.

It was mites, just hundreds of them crawling around the slide. It was so unpleasant that James immediately bleached the slide. Now, he’s not sure if that sample is the reason, but for days after, James kept sneezing and having to deal with a runny nose, all of which pointed to a potential mite invasion.

Fortunately though, our homes are not a utopia for dust mites. What they really need is dark, which is why they prefer to dig themselves deep into carpets and other soft things that give them space to burrow4. Dust mites have a harder time with materials like suede that are difficult to hide themselves in, and they usually avoid hard surfaces that are exposed to light.

So for James, his weapons against the mite invasion were clear: a vacuum, a mattress cleaner, and a strong UV light bulb. It is one of the more ignoble ends to one of our organisms. We don’t like to hurt them, unless they are hurting us.

But I am sure there’s a dust mite somewhere in our homes right now, settling into a soft, dark abode of its own, with hardly a care in the world for the battle that was waged to bring you this video. I don’t know what’s grosser. The idea of eating and pooping out of the same hole, or the fact that I’m constantly inhaling dust mite poop.

So let’s end on a happier note, a festival of feces, if you will, unfolding in the diverse strategies used by both single-celled and multicellular organisms to get rid of their waste. Ah, food, one of life’s great pleasures. Whether you’re an organism whose body can create its own food, or you make do with the consumption of other organisms, food sustains and connects life both large and small.

But we’re not going to talk about food today. We’re going to talk about what happens…after. No great meal comes without consequences.

Even photosynthetic microbes produce oxygen they don’t need, a byproduct that is quite useful to many other organisms, including us. And for organisms that have to digest their food to extract nutrients, well, there’s all sorts of stuff left behind when the process is done. In the world of the microcosmos, there are not only many varied tiny organisms.

There are also many varied ways to produce even tinier poop. And when we say “tinier,” we mean it. If you’re worried about being grossed out by this video, or you really want to make sure you’re not missing anything, just know that some of this pooping will require you to pay very close attention.

Some of this will probably look similar to macroscopic bowel movements, and some will seem less so. So let’s start with the more poop-like poop that comes from some of the multicellular members of the microcosmos, particularly the ones with digestive systems complete with specialized regions for digesting food and releasing solid waste. Our dragon-like friend the gastrotrich is doing the old scrunch-and-poop, probably after having finished a nice meal made up of bacteria or protozoa.

The food came in through its pharynx and was then sent through the intestine, a straight tube full of digestive cells. After, the remains continued their way to the end of the tube, where lies the anus--a microscopic gateway out of the gastrotrich and into the world. That’s all well and good for the more complex organisms, but how does one poop when one does not have a butthole?

Well, for some single-celled organisms, one becomes the butthole. Such is the case for amoebas. Their formlessness allows them to easily shift their shape around to take in food via phagocytosis.

The amoeba here happens to have found a nice meal made of a ciliate, which is now contained in a food vacuole filling with digestive enzymes. Those enzymes will eventually break the ciliate down, allowing the amoeba to absorb nutrients into its cytoplasm. But then what happens with the remains?

Well, as we see with this amoeba here, it gets released. Amoebas and other similar organisms use a process called exocytosis to send waste back to the membrane where it is removed from their body. You can see this again here in this Mastigamoeba, slowly gliding along the microscope slide while a round bit seems to come off up top.

It almost looks like it’s leaving behind a little piece of itself, which I mean, maybe it is. Ciliates have a more refined approach to pooping, which is to say they have the unicellular equivalent of an anus called the cytoproct. Ciliates gather food into an oral groove, and then consume it in the digestive vacuoles that travel through the cell.

Eventually, when the vacuole makes its way to the cytoproct, the contents get out, and there you have ciliate poop. The way organisms poop—the way microbes poop—are even more varied and wonderful than we’ve been able to show you here. But you might be wondering how you even get from something relatively simple like this ciliate releasing a small bit of waste to something more complex, like this tardigrade with its more compartmentalized approach to defecation.

Well the answer, of course, is evolution, but it’s an answer that leads to many more unanswered questions. Even within animals, the evolution of the anus—or in some species, the cloaca—is not well understood, as discussed in a 2015 review paper with the spectacular title, “Getting to the bottom of anal evolution”. While the anus itself is linked to the evolution of a digestive system, not all metazoans have a designated anus.

The hydra gut has only one opening, which means that the organism’s mouth has to pull digestive double duty and open back up to release the waste. And how that system fits in with the evolution of digestion overall is unclear. Much like the evolution of sex, anuses are a trait that appears and disappears in evolutionary lineages, reflecting the fact that they might be useful for some animals and less so for others.

The duality of the hydra’s mouth seems to be working out great for them after all. But having a digestive system with a distinct entrance and exit is pretty handy too. For one, it’s just a little less disgusting.

But it also keeps food flowing in one direction. And that means animals like the tardigrade and us don’t have to wait to finish digesting to eat more food. That would be annoying.

We just get to keep eating, all thanks to that evolutionary mystery that is the anus. In the end though, anus or not, everybody poops. Food, after all, might be one of life’s great pleasures, but all good things must come to an end.

And perhaps, with the microcosmos, we can find some pleasure in that as well. And that is the conclusion of our journey through the hind end of the microcosmos. It was beautiful, it was gross.

And I hope, most of all, that unlike the nutrients these organisms were getting rid of, this video was not a waste. Here on Journey to the Microcosmos, we have a complicated relationship with worms. On the one hand, they are gross.

They wiggle around. They end up in body parts. They cause disease.

All those things aren't great. On the other hand, they’re everywhere. You cannot escape worms, especially in the microcosmos.

And given everything we have said about how gross worms are, that doesn’t sound like good news. But there’s something to be said for ubiquity. Absence may make the heart grow fonder, but constant worm encounters can go a long way too.

We might even have a little affection for them at this point. So today, we’re going to look back on some of those many encounters and see what it is that makes worms so hard to ignore. Let’s start out though by talking about what a worm actually even is.

There are no ghosts in the microcosmos. There is no Halloween in the microcosmos. But if there were, this flatworm would fit in quite nicely with its spectral appearance.

It might be navigating its way through a field of other organisms, but it looks like it had three minutes to prepare for a costume party and decided to go with the classic “old bedsheet with eyeholes” ghost costume. And maybe comparing this flatworm (or even worms in general) to a ghost is kind of apt. Ghosts straddle two worlds, and so do worms, though in their case, instead of bridging the realm of the living and dead, worms manage to stretch between the micro and the macro.

And akin to ghosts, worms might seem like a sort of reduced form of life. After all, for a lot of us, worms are the wiggly tubes that we dug up from playgrounds. And of all of the worms, flatworms are perhaps the most simple of all.

But even digging into what it means to be the simplest worm makes the whole notion of simplicity seem ludicrous. So let’s start with the broader question: what are worms, other than wiggly, self-mating tubes? While we generally use the word “worm” to describe many different, unrelated, invertebrate, tubular animals that belong to a number of different phyla, there are three main phyla that people focus on.

The first are our flatworm buddies here, who are known more formally as Platyhelminthes. Then there is the phylum Nematoda, known less formally as roundworms. We’ve come across nematodes many times in our journey through the microcosmos, which makes sense given that they are one of the most abundant animals on this planet.

And last are the Annelida, phylum of our playground friend, the earthworm. While earthworms are a bit beyond the scope of the micro, if you will, we’ve also got this little Stylaria lacustris. At the tip of its head is the proboscis, an antenna-like structure that the worm uses for feeding.

And as we scan past the head, you can see that the length of the Stylaria’s body is segmented. Of the three phyla we’ve mentioned, annelids are the most complex. They have a closed circulatory system with tubes that transport nutrients and oxygen throughout their body, as well as a complete digestive system that has both a mouth to take in food and an anus to get rid of waste.

If we think of worms in general as a gut enclosed within a body wall, then that gut is a tube and the body is a tube, and you can imagine that worms are a tube inside a tube. Now what separates these different groups is the space that separates one of those tubes from the other. Complex systems inside annelids are due to what’s called a coelom, a tissue-lined cavity that sits between the tube of the digestive tract and the tube of the body wall.

Importantly, this cavity is filled with fluid, which facilitates the development of organs along with the transport of nutrients around the body. The development of the first coelom was a very big deal because coeloms don’t just allow annelids to have complex systems. They connect the seemingly simple organisms we’re talking about here to more complex animals—like us!

Because annelids and humans are both what is known as coelomates, or eucoelomates. We are animals that have coeloms. So, thank you to the ancestor that we share with earthworms who developed the first coelom, who allowed more complex organisms like you to exist.

Nematodes, on the other hand, are what are called pseudocoelomates. They still have a coelom-like fluid-filled cavity, but the cavity isn’t lined with tissue like you find in true coeloms. Other microscopic pseudocoelomates include gastrotrichs and rotifers.

This pseudocoelom is a slight but significant difference in nematodes when compared to annelids, corresponding to a slightly less complex body plan, though nematodes still do have a complete digestive system. The flatworms are the simplest of all. Starting with the coelom: flatworms don’t actually have one.

They’re considered acoelomates. That area between the gut and body wall where annelids and nematodes have a cavity full of fluid is instead full of tissue. This has a few consequences for the flatworm, but one of the biggest ones is that this area can’t support the development of specialized systems that you see in other animals.

The lack of coelom imposes limits on the flatworm’s body and lifestyle, reducing both its complexity and size. But that does not mean the flatworm lives a simple life. Of the three worm groups, flatworms are the most like a tube within another tube, except that their outer tube is much less cylindrical.

The “flat” in “flatworm” is actually an adaptation: without a circulatory system or respiratory system, the worm relies on diffusion across its outer membrane to supply cells with oxygen. Being flatter gets the cells closer to the outside and better positioned for diffusion. And without the ability to build a complete digestive system like those found in nematodes and annelids, flatworms have to adjust to life without that most under-appreciated body part of all: the anus.

They take in food through one opening, digest it in their gut, and then spew the waste back out from that same opening. So unlike other organisms who have separate entrances and exits for their food, the flatworm must wait to finish digesting before it can eat again. But this is not the only way flatworms can get rid of waste.

Their body is lined with a special type of cell called a flame cell that also gets waste out of the worm. While it may not have the elegance of an interconnected set of organs, this system is its own form of complexity. Basically, every flame cell is one single-celled kidney, allowing the worm to just...ooze waste from its skin.

And this is just scratching the surface of the biological intricacy flatworms are capable of. Like many of their more involved worm counterparts, flatworms can reproduce both asexually and sexually. And when reproducing sexually, they can mate with another flatworm, or even with themselves--both making and fertilizing their own eggs.

There’s also those flatworm species that can regenerate, a trick that our more developed bodies are, alas, not capable of. Even the many weird ways they move make the word “wiggle” feel reductive. Our perception of simplicity across nature is built on comparisons.

Compared to us, an earthworm may seem simple. Put that earthworm next to a flatworm, and suddenly the earthworm represents a gigantic biological advancement. But then put that flatworm next to a bacteria, and suddenly it contains multitudes, it is a universe.

That flatworm is a marvel, an almost unbelievable testament to the power of evolution. So we started with the simplest worm of all. And if you’re someone who is skeptical that worms can be interesting, then maybe watching a simple, jiggly tube was not enough to sway you.

But nature has a way of building on simplicity, creating fascinating creatures out of even the most basic of blueprints. And that brings us to the next worm we’re going to focus on… the ones we affectionately call “polka-dotted vacuum worms.” We recently did an episode about nematodes, the phylum of worm that outnumbers just about every animal on this planet. Now, it's not the most striking of animals, but the nematode has had a few distinguished scientific decades, thanks to its many uses in laboratories.

So as far as worms go, the nematode seems to dominate much of our scientific understanding. But worms, despite their seemingly simple bodies, are a diverse bunch. Which is why we thought for today it might be fun to visit with a less famous worm and like one of those relatives that you don't really know very much about But every time you see them, there's a new strange story to unpack They are the Aeolosomatids, a family of freshwater worms.

The ones that you see here are invaders. They showed up uninvited in a blepharisma culture that James, our master of microscopes, has been taken care of for a long time. And while Aeolosoma worms are you know, worms, they are in a different class of worm because as we have seen before, there are, in fact, many ways to be a tube.

Where nematodes are roundworms, Aeolosoma are segmented, placing them in the Annelid phylum, along with earthworms and leeches, Aeolosoma are usually several millimeters in length, their bodies divided into more than ten segments that you can see scrunching up and expanding as the worm wiggles its way through the microcosmos. The Aeolosoma are striking to look at. You can see their organs through their transparent bodies, and as it moves, bundles of long bristly hairs wave along the side of its body.

Those hairs mark the Aeolosoma as a specific type of an annelid called a polychaete, or bristle worm. Some bristle worms are found in unusual places like hydrothermal vents, but our Aeolosoma come from a much more mundane home. They're usually found in bodies of fresh water where they'd like to crawl among the leaves and algae that settle at the bottom of the water.

And inside their bodies are colorful gland cells, though no one is really sure what those cells exist for or why they have their particular colors. And some species, the cells are green and others they're yellow. And sometimes, as with our worms, they're red.

The final result is a worm that looks a little like it ran into a porcupine while also having caught chicken pox. While there are some Aeolosoma species that reproduce sexually, most reproduce asexually dividing to form a copy of itself. The Aeolosoma creates its clone at its end, linking the old and new versions of itself like a chain.

You can see the new Aeolosoma here, looking like it's attached to the other’s butt because, it's attached to the other’s butt. And this chain can keep going as the Aeolosoma keeps dividing, adding more worms that are connected together so that the final length of their combined bodies sometimes reaches around ten millimeters total. That's ten millimeters of clones combined to create one giant mega worm until eventually the chain breaks and they all go their separate ways.

So when James found these worms invading his samples, you'd think maybe this would be an exciting find. Here is a culture full of bristled, polka-dotted, chain-forming clones. What could be more exciting!

Well, as wonderful as they are to look at these invasions are not ideal because they are also essentially vacuum cleaners. Their mouths are lined with cilia that wave around and help the worms suction up bits of plant and animal debris. When they're in a pond, They like to crawl across leaves and algae for their meals.

But when you find them in bottles of ciliate cultures, you've been lovingly maintaining, that's when things get a bit dicier. Because Aeolosoma will eat just about anything, including each other. Indeed. in one very dramatic scene documented in 1901 scientists observing the species Aeolosoma tenebrarum described the way these chains of worms would twist up in each other, creating a writhing, tangled ball of worms that would stay stuck together for long periods of time.

And when the scientists pulled these balls apart, they usually found at least one worm that had been partially eaten. I'm sure the etiquette around cannibalistic frenzies varies, but for most animals, getting eaten by another member of your species would seem, at the very least, a little rude. But when you're Aeolosoma, it's not that big of a deal.

Honestly, it's not much more than an inconvenience, because if a part of it gets eaten, it can always regenerate. In one case, the scientists watching these balls of worms found that one worm had its head eaten. But in about three days it was able to make a new one.

It would probably have taken less time to regenerate other parts of their body- heads seemed to take the Aeolosoma a bit longer, perhaps because of all the complex parts that need to be rebuilt. And the Aeolosoma can regenerate even when it is cut into multiple segments. This superpower has made one species called Aeolosoma viride particularly interesting to scientists.

And it's not just that they can regenerate. After all, as incredible as this ability is, there are plenty of other animals that can regenerate as well. But scientists aren't just interested in how animals regenerate.

They also want to know how those regenerative abilities change as the animal gets older. That's a difficult question to study because as you might expect, self-healing animals have often, pretty long lifespans. So it's a challenge to wait years or even decades to study how their ability to regenerate changes with the wear and tear of aging.

Aeolosoma viride however, has a lifespan of only about two months, which means it goes from young to old on a manageable timescale for scientists cycling through experiments. And that makes it a useful organism to observe how that capacity to rebuild itself changes as the worm ages. But as useful as regeneration is for survival, it is not the only tool the worm relies on.

After all, not all dangers can easily be patched up by rebuilding body parts. Sometimes the worm has to preempt dangerous conditions, and for that it turns to the cyst. In nature, the worms will likely begin forming these cysts in autumn, when the water gets cold and begins to fill with the remains of decomposing life.

And as the temperatures continue to fall, the worms begin to slow down, crawling to areas full of delicious debris for them to stock up on, and eventually, the worms begin to secrete a mucus, creating a gooey shell that then hardens into a cyst. You can see the granules of red pigment swirling around as the worm moves inside. Some of that activity might be the peristaltic movement of its intestines, but it's also possible that the warmth of the microscope lamp is causing the worms to stir as well.

And in their ponds, when warm weather comes, the worm will get ready to emerge from its encased hibernation, using its head to push at the hardened case of its cyst until it manages to poke a hole through from which it can escape. It can take a worm anywhere from 30 minutes to several hours to make its exit. And if there's a thick coating of bacteria on the cyst, it may even take the worms several days.

And from there well, it is a life of suction, feeding and chain link clones and regenerating. Perhaps not normal to us, but what's normal anyway? Especially when you're a worm.

Now that we’ve seen a few worms, have you ever wondered what it would be like to live inside of one? Because I certainly haven’t. But there is a ciliate that does like to live in the guts of worms, so in our next video, we’re going to learn more about why they do that.

You’ve heard some worm horror stories, right? We were looking some up just for this episode and came across a recent headline from ArsTechnica that read, “Army of worm larvae hatch from man’s bum, visibly slither under his skin,”. And if that’s not enough to terrify you, and make you feel very uncomfortable there’s always the stories of painful stomach cramps or diarrhea or nausea that eventually turns out to be caused by some worms that have taken up residence in someone’s intestines.

It is terrifying and wild to think of something so much smaller than us causing so much havoc. So, as we watch the cilia lining a worm’s gut beat its own soothing pattern, wouldn’t it feel like, almost like, a little bit of justice if this sight wasn’t so peaceful? If worms had to worry about their own guts being taken over by a parasite?

If you’ve found yourself in this position, seeking schadenfreude over a worm, well we have some good news for you. The worm you see in the middle of this tank is currently hosting this strange fellow, called a Radiophyra. James, our master of microscopes, had been on the hunt for the Radiophyra after seeing this: two radiophyras linked together in a chain as one divided to make more copies of itself.

It had come from the inside of one of the worms he’d been watching, when he’d accidentally squeezed a worm a bit too hard under the coverslip and caused the ciliate to pop out. Radiophyra belong to a general group of ciliates called Astomes, or astomatid ciliates. We’ve talked about ciliates a lot on our channel, which means that if you’ve been watching us for a while, you may have picked up on the fact that from time to time, we have said that most ciliates have an oral groove, that opening lined with cilia that sweep bacteria and algae and other tiny bits of food into the organism.

We’ve seen that oral groove at work in ciliates like stentors and paramecium, functioning as the ciliate equivalent of a mouth. But as we have always said most ciliates, you will have inferred, that this does not mean all ciliates. And if you are looking for an exception to the rule, astomes are that exception.

Astomatid ciliates are diverse, but they are unified by one shared feature, or rather, they are unified by their lack of one shared feature, a mouth. And they don’t need a mouth because they have something even better. They have worms.

Astomatid ciliates do parasitize animals other than worms. Some live inside mollusks, others inside leeches or even in amphibians. But they are most commonly associated with the guts of annelid worms.

So when James found his Radiophyra, he decided to see if he could find more of them in the other worms that were in his samples. And that meant that our master of microscopes had to become a master of worm surgery, dissecting them so he could draw out the ciliates living within. In–side this particular aquatic worm were these astomatid ciliates.

From a distance, they also look like worms. But as you get closer… And closer, their shape becomes more definite except for the massive amounts of fluff around them, a dense cloud of cilia beating away. Unfortunately, there isn’t a lot of research on this ciliate.

In fact, there isn’t a lot of research on astomatid ciliates in general. They just aren’t destructive enough or common enough to have become either a necessary or convenient research subject. In fact, it’s not even clear whether or not we should call them parasites.

Modern day papers will sometimes refer to them endosymbionts instead, because we don't know a lot about whether astomatid ciliates are doing much to their worm hosts, bad or good. But the worm gut does plenty for the astomatid ciliates. At one point in time, the ancestors of these ciliates did have mouths.

But as they found their way into worms, and specifically their guts, those oral grooves became less and less necessary. Instead, the ciliates could rely on a form of feeding called osmotrophy, where they simply absorb nutrients from their surroundings through osmosis. Instead of taking in larger bits of food through their mouths and breaking it down themselves, astomatid ciliates could just take advantage of the worm’s digestive system to do all that breaking down for them.

As the worm’s digestive enzymes break down complex molecules into simpler forms  that can travel through their own intestinal walls, some of those nutrients would just go feed the astomatid ciliate instead. These ciliates will actually sometimes be picky about making sure they’re in a particular spot within their host’s intestinal tract. And once they’ve found the right spot, the ciliates hold themselves in place with organelles that range in shape, some use hooks, other spines, or maybe even suckers.

The flat shape of the ciliates helps them stay pressed to the epithelium of the intestines. So while these ciliates may not need a mouth anymore, they have found other traits necessary to their survival. Astomatid ciliates are found in hosts from all sorts of environments.

Some live in soil. Some live in ponds. Some even live in ocean waters.

And scientists are using the general tools available now to try and piece together how host and endosymbiont have shaped each other. We can see some of that intertwined story in the ciliate’s mouthless-ness, but the specifics of that evolutionary change are laid out in their DNA, as are the other more hidden parts of that shared history. Now we cannot know whether worms have any feelings about their guts being home to another organism.

They likely don’t have much choice in the matter, and they also likely aren’t prone to emotions like resentment the way we might resent a worm parasitizing our bodies. But if they were to have any feelings on the matter, we want to offer this one last detail about astomatid ciliates: many of them have their own endosymbionts as well, bacteria that live inside them with perhaps their own history tangled up with their ciliate host’s story as well. Now we couldn’t find those bacteria in our parasitic friends, but we like the idea that somewhere out there are these nesting dolls of endosymbiosis: an organism that is an organism but an organism that is also a home buried within other homes.

When we say that worms are everywhere, we mean it. Even in the world of science, worms are widespread with researchers turning to the nematode Caenorhabditis elegans as a model to study all sorts of questions. And in the process, scientists have written some lovely things about the nematodes that make their work possible.

So in our next video, we’ll pay tribute to a worm that has shaped our world in so many ways. When it comes to the muses of the animal kingdom, the nematode seems like an unlikely well of inspiration. There may be poems about tigers burning bright, and epic fantasy tales featuring giant birds.

But nematodes have two obvious factors working against them achieving that kind of mythical status.

First: they are worms. And let's not generalize too broadly, but their simple tubular bodies are just not what most people think of when it comes to the beauty of the natural world.

Second: most of them are really tiny, their length measured in millimeters. It’s hard to be inspired by an animal that is likely to escape one’s notice entirely. Though we should note that there are nematodes that can grow quite long—the largest discovered was found in the placenta of a sperm whale, and it measured between 8-9 meters in length. But even that seems more likely to inspire more nightmares than art.

In 1914, a scientist named Nathan Cobb wrote the following about nematodes: Nematodes do not furnish hides, horns, tallow, or wool. They are not fit for food, they do not produce anything fit to eat; neither do they sing or amuse us in any way; nor are they ornamental —in fact, when they are displayed in museums the public votes them hideous. Judging from that quote, Cobb seems to understand the common consensus on nematodes, which is that if there is to be a consensus, it is not a positive one.

But Cobb wrote this as part of a 34 page paper on nematodes titled “Nematodes and their Relationships,” which wonderfully documents his fascination with the worm and argues for their importance to our understanding of the world. So clearly, he found something inspiring about the nematode. But there is another quote from this research paper that you may have even heard before, it is pretty famous in nematode circles.

And The quote is as follows: In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. Cobb paints a world overrun by nematodes, an image created by his own experience with studying them. But when he wrote this, he likely didn’t have the means to estimate just how many nematodes there are in the world.

But in 2019, researchers estimated that there are about 57 billion nematodes in the world for every human in the world. In addition, their total biomass is about 300 million tons. And that’s just for the soil nematodes.

When you take into consideration nematodes living in freshwater or marine habitats or inside of other animals, the numbers only go up from there. Nematodes are considered to be one of the most abundant animals on earth. But the beauty of Cobb’s writing on their abundance is that he doesn’t just capture the notion of the numbers.

The world he describes isn’t just a mass of worms that we’ve been allowed space inside of. The world he describes—the one that we live in—has its forms and landscapes sculpted and cultivated by worms, their bodies shaped around and inside trees and plants and animals. Of course, not all of those links between nematodes and other organisms are always so pleasant.

In “Nematodes and their Relationships,” Cobb mentions that he once removed over 40,000 nematodes from the stomach of a wallaby. And if that isn’t daunting enough to imagine, there are always the ancient texts that date back thousands of years, documenting intestinal worms found in people. There are also the nematode eggs that have been found preserved in mummies and in fossilized poop.

In the 19th century, scientists were able to show that the parasite Ascaris lumbricoides which resides in the intestines and produces eggs that are passed on through poop finds its way into people who have ingested their eggs on contaminated food. To prove this, a scientist named Giovanni Battista Grassi infected himself with their eggs and then hunted for those eggs in his feces. A peculiar aside here this is not the first time we’ve described a scientist hunting through his own poop for evidence of the microcosmos you can watch our Leeuwenhoek episode for more tales of this type.

Now there are plenty of other nematode parasites that have their own unique life cycles and horrifying effects. Some worms are transmitted through the soil, others through insects. Some inflame the limbs, others trigger massive blood loss.

Nematodes can also parasitize plants, attacking the roots or stem or flowers, sometimes destroying entire crops in the process. And so with such a destructive path, the nematode feels like an enemy, something we study only to understand how we can fight it. But Cobb asks a simple question in his writing: What would be our conception of the insect group as a whole if our knowledge was largely confined to these simple and degenerate parasitic forms?

In the case of insects, we have enough experience with them to know that whatever squeamishness they inspire, they are also integral parts of our world. And likewise most nematodes are actually free-living species. It’s just that the parasites have come to dominate both our imagination and our knowledge because of their proximity and their consequences for our health.

But those massively abundant soil nematodes they are vital to our world, feeding on bacteria and fungi. In the process, they release nitrogen back into the soil, sustaining other organisms and plants that may use it. A healthy soil is full of these free-living nematodes, whose presence reflects the overall diversity of microbes around them.

The information is all there, it is up to us to uncover their secrets. Perhaps this why Cobb ended “Nematodes and their relationships” with this call to arms for the field of nematology: They offer an exceptional field of study; and probably constitute among the last great organic group worthy of a separate branch of biological science comparable with entomology— nematology. We can’t speak to any claims that the nematode is the quote “last great organic group” worthy of its own branch of study.

But we do feel that he has been vindicated by the fact that one of the most popular model organisms used today is a free-living nematode: Caenorhabditis elegans, better known as C. elegans. While the worm is found in soils all over the world, what it’s perhaps best known for is the life it lives in labs. While scientists had described various aspects of C. elegans life early in the 20th century, it wasn’t until 1965 that the scientist Sydney Brenner turned to the worm as a model organism, relying on its fast life cycle, small size, and capability to produce more than 1,000 eggs a day.

There were those who thought the worm’s simplicity would render it useless in studies of morphology and behavior. But time and technology has turned C. elegans into a molecular muse. It has taught us about how life develops, how human diseases progress, and how cells die.

When Cobb wrote, “My experience in this matter makes me very confident in saying that professors of biology could do far worse than to introduce into their courses a more careful examination of these creatures,” he didn’t know that one day, many of those professors of biology wouldn’t just be introducing nematodes into their courses. They would be introducing them into their labs. When Cobb died in 1932, scientists didn’t even know how genetic information was encoded in DNA.

More than 50 years later, C. elegans became the first multicellular organism to have its genome sequenced. And as it continues to teach us about our world, NASA has even sent them to space so we can learn about how life ages outside of this planet. So maybe the nematode is not the muse we expected.

But over the past century, they’ve become one nonetheless. And perhaps that is the most fitting testament to what a muse often actually is, a surprising source of inspiration that seems to come from nowhere, and then permeates until you see it everywhere. We’ve been on a journey in this compilation through knowledge and a growing appreciation for worms.

And it brings us to our last video, where we went from not just being resigned to the presence of worms, but actively seeking one out and dealing with all the surprising challenges along the way. We have spent most of our journey through the microcosmos seeking out the organisms that are too small to see with the human eye. The bacteria, the ciliates, the tardigrades.

And Part of what makes them so exciting to find is that they are so tiny. Every moment we spend with one of these organisms is a peek into something exceptional in our experience of the world, and it’s the result of how much work James, our master of microscopes, has to put into hunting down as many microbes as he can. And sometimes, that effort requires a lot of persistence.

Take the creature we’re going to focus on today: the bristle worm. This has been one of the white whales for our channel for some time. And as you watch it, you can perhaps understand why we have been searching so hard for it.

It’s got the body of a pipe cleaner with the head of a cartoon dragon. And maybe you also understand that just because we’ve been wanting to find one of these worms, that doesn’t mean we are guaranteed anything. After all, one of the things you have to accept about the microcosmos, and microbe hunting, is that it is a big world full of tiny creatures, and it can take a while to find some of them.

The fact that we are showing you one of those bristle worms right now spoils the twist we would usually build into, but, surprise, we found a bristle worm! The real twist is that this was not the first bristleworm we found. This was the first bristle worm we found.

Twenty centimeters of segments and bristles climbing up the side of a tank before burrowing back beneath the sand. Unfortunately, that’s just about all the video we got of that bristle worm. James spent so much time trying to find this bristle worm again that he started to feel a connection to it.

So he decided to give it a name: Gunther. James spent hours trying to catch it without hurting the worm. But Gunther has hundreds of appendages that can grab onto sand, and James didn’t want to accidentally snap the worm in half with his tweezers.

So instead of showing all the features of Gunther that we wanted to show, we’re going to show clips of this bristle worm instead because it kind of resembles Gunther, except a lot tinier. Gunther is hopefully still somewhere in that tank, living a nice life in the burrow it dug for itself. But it’s hard to know exactly where in the tank Gunther made its home because it would only show activity in the dark.

The moment James tried to turn the lights on and watch the worm in motion, it would vanish again under the sand. And as it became harder and harder to find, James kept hoping that one day, something would bring another bristle worm to him. And then, one day, James received a package from a coral farm containing sediments and other things that might contain some interesting organisms.

We talked about some of those organisms that James found in our last episode. But in addition to all of those organisms, James found several species of his long sought after bristle worm. Bristle worms are also known as polychaetes, and they’re part of the segmented worm phylum known as Annelids.

The name “polychaete” translates in Greek to “many hairs.” Those stiff hairs are called setae, and for most polychaetes, they’re attached to paddle-like appendages called the parapodia that branch off each segment of the worm. Our giant friend Gunther most likely belongs to the order Eunicida, but its microscopic look-alike is more unknown to us. But we can imagine that it spends its life crawling around the sand and feeding on algae, or whatever else it can take a bite out of.

You can see this one moving its mouth in slow motion, like a weird pair of pincers inside clamping down on something, and the movement is even more dramatic in full speed. This probably looks like a bunch of tiny individual orange worms tangled together. But it is, in fact, a worm belonging to the genus Cirratulus that can get to around 12 centimeters in length.

And along with its distinctive color, the worm is easy to spot in wet sandy mud because of those threads you see waving across your screen. Some of these threads are tentacles, but others are actually gills. And in the water, those threads seem to float serenely.

But this effect is lost on land. In a paper from the beginning of the 20th century titled “Notes on the Ecology of Cirratulus tentaculatus,” the author wrote, “When withdrawn from the mud Cirratulus presents an exceeding limp and bedraggled appearance.” This worm has a more imposing appearance but also a funnier name. It belongs to the terebellid order, but it’s known as a spaghetti worm.

These tropical worms live in sand, building tubes out of gravel and limestone to live in. Their tentacles spill out from the tube, sometimes extending as far as a meter to gather building materials and food for the worm. To reproduce, spaghetti worms will release their eggs and sperm into the water, but only at night and sometimes even without other males or females around.

They seem to do this on a lunar cycle, with a limited two week window for these gametes to release and find each other. That seems like an extraordinarily chance-y way to reproduce, but it seems to work for this worm. These are the bristle worms we were able to find from our coral farm samples.

And so it seems like our hunt for this white whale might be over. Except, the more we’ve been reading about bristle worms, the more we wish we could find even more. Because these are an animal whose existence seems designed to inspire lists of fun facts and bizarre trivia.

There are thousands of species of polychaetes, and they all seem to have something remarkable or weird about them. Some spend their lives in tubes that stick up from the sand, using their parapodia to paddle water through the burrow. Others have managed to carve out lives near hydrothermal vents.

Some sound like they’re straight out of a horror novel, growing up to ten feet long or dining on the bones of decomposing animals. And some even have eyes on both ends of their bodies. So knowing that all of these different species are out there, how could we ever end our quest for the bristle worm?

There are a lot of creepy creatures in the world, and the microcosmos is no place to escape them. For every adorable tardigrade waving its stubby legs around, there are also countless nightmarish organisms lurking. And perhaps one of the most unsettling creatures to us here on our Journey to the Microcosmos is the mite.

It’s not enough for these tiny arachnids to remind us of every nightmare we’ve ever had involving spiders. These organisms also produce different compounds that manage to be unsettling in their own unique way. So today, we are going to revisit some of our previous videos about mites, following along through the varied lives different species live, and the…well let’s call them gifts …that they leave along the way.

And we’ll start out with the mites that might be sharing a room with you, right now. One of the most frequent questions we get asked  on Journey to the Microcosmos is how James,   our master of microscopes, stays safe  while gathering and studying samples.   After all, as beautiful as the microcosmos is,  there are plenty of parasites and pathogens in   there that we’d prefer to keep under  the microscope and out of our bodies.  But while James has simple and effective  guidelines to protect his health—like   making sure he always washes his hands before and  after handling samples, and never venturing into   a pond with an open wound—there are times  when those measures don’t quite cut it.   These are the times when no matter how much  you protect yourself outside or inside,   the microcosmos gets the best of you. These are the times of the dust mite.  Now we don’t usually start these videos with  a warning, but we’re going to do that today.   While working on this video, at least 4  members of the Journey to the Microcosmos team   found themselves having a bit of a  crisis about how clean their homes were.  If you’ve found yourself sneezing and wheezing and   running around miserable with a runny  nose because of dust, this arachnid   is probably the culprit—though perhaps  not in the way you would expect.  Dust mites are small in the grand scheme of  things, but large in the microcosmos, hitting just   around 300 micrometers in length.

Their bodies are  decked out with four pairs of legs ending in feet   that act like suction cups as they wander in their  quest for three things: water, food, and darkness.  Dust mites love humidity because their  lives depend on it. They don’t drink water   so much as they osmosis it into them, relying on  a salt-filled gland near their mouth to absorb   water vapor from the air around them. So if their  surroundings become more arid, an adult dust mite   will wither away until it eventually dies.

This is a challenge for dust mites that live   in places with seasons, where wet and rainy falls  give way to dry winters. To survive those shifts,   they rely on the resilience of their  younger brethren—the nymphs—whose   forms are able to withstand the lack of moisture. The seasons of the dust mite’s life was one of the   important clues that helped scientists in the 20th  century understand their role in dust allergies.   Scientists identified dust as the source of some  kind of allergen in the 1920s, but they were not   sure what it was exactly about dust that did it.  They just knew that people all over the globe   suffered from dust allergies, and that these  allergies were often seasonal, peaking in the   autumn, particularly after warm, humid summers2.

And other possible sources, like animal dander   or mold, just didn’t quite fit right with the  seasonality of the allergy. But in the 1960s,   scientists in the Netherlands and Japan realized  that the dust mite might be the culprit.   They’re found in large numbers in the dust that  lines the unwashed and untouched corners of our   homes, their populations peaking in time with the  runny noses and bleary eyes of dust allergies.  But while experiments confirmed that mites  really were the source of people’s sensitivities   to house dust, it wasn’t clear what made them  so special. So, we’ll get to that in a bit...  James often finds dust mites in the humidity  chambers he uses to keep his microbes alive.   These chambers are made up of a  dish lined with wet toilet paper,   which is just about all a dust mite needs to feel  right at home—especially when there’s potentially   some delicious microbes to munch on.

So to keep  those other microbes from becoming a mite meal,   James has to regularly clean out  and disinfect the humidity chamber.  Now, that cleaning might get rid of the  dust mites in the humidity chamber, but   the problem is that our houses are full of  food for them because our houses are full of   us—of flakes from our skin that shed and gather  all around and sustain the invisible mite.  And this is a problem not just because it  means the mite can thrive in our homes.   It’s a problem because food means poop. Over  the course of its life, a mite will produce   about 1000 pieces of poop that are roughly the  size of a grain of pollen. Inside those bits of   fecal matter are enzymes that help the dust mite  eat its own poop and get nutrients that it might   not have gotten the first time around.

But should the dust mite choose to not   revisit its prior meals, the feces will float  around the room, attached to other particles   until eventually they settle down—perhaps on  a pillow, or on a pet’s bed, or a car seat.   It’s like we’re living in a gigantic snow globe,  except that the snow... is dust mite feces.  In 1981, researchers confirmed the bad news about  this animal’s poop— people are allergic to it.   To be more specific, they confirmed  that dust mite poop contains specific   proteins that many people are allergic to. So if you’re airing out some sheets and you start   sneezing, what you might actually be reacting  to is dust mite feces flying around the air.  Now, we apologize for this mental image but it  is reality, and it has embedded in our heads,   so we have to share it with you as well. But  we can offer what might be a small comfort:   there is another allergen from dust  mites that has nothing to do with poop.  Dust mites have a fairly lengthy mating process,  sometimes taking up to two days to finish—this   is a pretty lengthy time for any organism, but  especially for an animal that’s only got about   100 total days to live.

Over the course of their  life, the female dust mite will lay up to 80 eggs,   which hatch into larvae that then go through  several different stages of development   before becoming adults With each passing stage,   the dust mite sheds its exoskeleton, leaving  behind its youth. And that exoskeleton provides   some of the other allergens for people react to. But though that is something people are sometimes   allergic to, it really is mostly just the poop  that sets off people’s allergies.

So I guess there   really wasn’t that much comfort there after all. And even for a trained and cautious   microbiologist, dust mites can  become an unwelcome surprise.  Once, James brought home some samples taken from  a water dish that his neighbor left outside for   their cat. The surface was covered in tiny round  things, and James thought they might be rotifers.  But when he looked at the surface of the scum  under the microscope, there were no charming   rotifers to be found.

It was mites, just  hundreds of them crawling around the slide.  It was so unpleasant that James  immediately bleached the slide.  Now, he’s not sure if that sample is the reason,  but for days after, James kept sneezing and   having to deal with a runny nose, all of  which pointed to a potential mite invasion.  Fortunately though, our homes are not a utopia  for dust mites. What they really need is dark,   which is why they prefer to dig themselves deep  into carpets and other soft things that give them   space to burrow4. Dust mites have a harder time  with materials like suede that are difficult to   hide themselves in5, and they usually avoid  hard surfaces that are exposed to light.  So for James, his weapons against  the mite invasion were clear:   a vacuum, a mattress cleaner, and a strong  UV light bulb.

It is one of the more ignoble   ends to one of our organisms. We don’t like  to hurt them, unless they are hurting us.   But I am sure there’s a dust mite somewhere in our  homes right now, settling into a soft, dark abode   of its own, with hardly a care in the world for  the battle that was waged to bring you this video.  If you have not abandoned this video yet to go clean your room, we do appreciate it. Because next up, we have water mites, who are perhaps less of a scourge to us than dust mites, but who are still capable of producing their own strange compounds with… uncomfortable implications.

For those of you afraid of spiders, the  microcosmos probably seems like a safe haven.   When you’re scanning through a microscopic  ocean full of single-celled organisms,   the likelihood of a spider crawling  across your screen feels comfortingly low.   The few times we’ve seen a spider under the  microscope, it’s because we put it there.  But safe as the microcosmos might  seem from a surprise spider attack,   it would be misleading to pretend that it’s  completely free of spider-like sightings. Because   even at this small scale, you could find yourself  subject to an ambush of the arachnid sort.  These are water mites, relatives of spiders  in that they are also members of the Arachnid   class. And the resemblance is uncanny,  especially with the eight legs waving around.  But that detail aside, there is something about this water mite that looks   more like a cartoon spider, something  you might have doodled in a notebook.  For one, there’s the eyes, those simple dots.  Water mite species usually have two pairs,   which act mainly to detect light.

It’s much  less intimidating to stare into the eyes of a   water mite than into the eight eyes of a spider. Another reason for the cartoon spider-iness of   some water mites is down to a defining aspect  of their shape. Spiders have segmented bodies,   with an abdomen that is distinct from  the cephalothorax that holds its head.   But water mites aren’t segmented.

Instead,  those different parts are all fused together.   In the case of this particular species, the  final result is round and kind of adorable.  So now we’re going to disrupt that cuteness by  bringing up a creature we’ve talked about before,   one that’s more closely related to water mites  and one that does not evoke any sense of cuteness.   We’re talking about dust mites, which are part of  the same superorder of mites called Acariformes.  If you haven’t watched our episode about  dust mites before, you should check it   out after watching this video, especially if  you’re looking for motivation to clean your   room. Because without spoiling too much, the main  thing to know about dust mites is that their poop   is, it’s not great. I’m bringing that up now   because while water mite poop might not evoke  the same concern that dust mite poop does,   it is still fascinating.

Their excretory system  is made of a large tube connected to a pore.   The tube takes in waste from the mite’s version  of blood, called hemolymph. And in the tube,   the waste is stored as yellow or white  crystals. To clear out its storage of crystals,   the mite simply moves its muscles to push  everything out through its excretory pore.  From the studies we’ve found on other mites  with similar systems, our best guess is that   these poops are guanine crystals, produced by  the mite’s own body as it breaks down nutrients.  Now crystal poop is weird.

But the weirder thing is that crystaline poop is not the   weirdest thing that water mites excrete. But before we get there, let’s talk about just how   red some of our water mites are. They’re so red  that they seem like walking drops of blood.

They are not actually chock-full of blood though; the red color is likely just some kind of carotenoid   that turns the water mite crimson. And for a time, scientists believed   that the red color was a way for  these species of water mites to flash   a warning signal to fish, a sort of “Do not eat  me” expressed solely through threatening hue.  But like why? Why did scientists think fish would  heed this warning anyway?

Why would a fish   bother to listen to a water mite  when it could be eating instead?  Well, many water mite species have an  approach to warding off predators that   is…sticky. Water mites have little globe-shaped  organs dotting their bodies called glandularia,   each with their own little opening and a little  bristle-y hair called a seta. When something, say,   a fish, brushes up against that hair, it triggers  the glandularia into releasing a milky fluid into   the water, where it mixes into a tackier,  more viscous substance.

And for predators,   the idea of having to wade through some gloppy  mess to get to your food is probably unappealing.  So when the scientists saw, in the context of their own  experiments, that fish seemed to quickly stop trying to eat water mites, they decided  that the red coloring was the water mites’   way of telling the fish, “We’re the sticky prey.” But while it’s possible the fish do learn not to   pick out these water mites for food,  the idea that the red color exists   specifically as a warning to predators does not  hold up to the basics of how these particular   species live. Red water mite species tend  to be found in transient pools of water,   places that fish don’t often find themselves.  So the likelihood that they would have evolved   this trait for the purpose of warning fish is low.  It’s more likely that the pigments are a way of   protecting them from ultraviolet light. It just maybe happened to also protect them from fish as well.  And the sticky fluid that makes them unappealing to  fish also has another use: reproduction.   In one genus called Arrenurus, the males use it to   hold the females still during copulation.

This is part of a wider spectrum of weird   reproductive behaviors. On one end, there are  the species like the ones we just mentioned,   which engage very directly and  physically with each other.  On the other end of the spectrum, there are water  mite species where the sexes don’t feel the need   to interact at all to make reproduction happen.  They don’t communicate, they don’t touch. The   males simply deposit spermatophores somewhere for  their female counterparts to find them and pick them up.  And then, somewhere between the directly  copulating water mites and the cold shoulder   water mites, there are the dancers.

They’re  sort of like the second group in that the males   drop off their spermatophores. But then they  actually stick around, mostly so they can draw   females in and help them find the spermatophores. And this is where dancing comes in.

Sometimes this   involves vibrating their legs  to mimic their favorite prey,   drawing the attention (and aggression) of a  wandering female. Other times though, this means   waltzing together as a pair, moving in circles  with careful pauses to pick up the spermatophores.  We obviously don’t have video of that or  we would be showing it to you. But at some point,  some scientist watched this happen.

Someone  watched the dance of the mating water mites,   and then they carefully documented it.  Just like someone saw the mite-averse fish,   and the crystal poops, and the many other strange  things that make up the lives of water mites. So far, we’ve been describing the ways that mites make things that make other creatures uncomfortable. But to cast them solely as the excretion villains of the microcosmos would be unfair.

In fact, if you’re a fan of cheese, there’s a good chance that you might actually be a fan of mites too… even if you don’t know it yet. In May 2013, a shipment of around 1.5 tons of cheese was refused entry into the United States. The cheese in question was a spherical, orange variety known as mimolette that’s sometimes compared to cantaloupes in looks, and lemons in taste.

There’s nothing super startling about that description— nothing to suggest, at least, that this is a suspicious cheese. But according to the Food and Drug Administration, this shipment of mimolette had a problem: it had mites. If the idea of mites on your cheese makes your skin crawl, we have good and bad news for you.

The good news is that most cheeses are not usually filled with so many mites that the FDA decides to take action. The bad news? Well…this mite was scraped from the rind of a Milbenkase, a cheese from Germany that is also known for its lemon-y flavor.

And if you like milbenkase and mimolette but hate mites… the bad news is, you’ve been eating a lot of mites. The footage we’re showing you today was sampled and recorded by Chloe Savard, one of our guest masters of microscopes. The last time we featured her videos, we were looking at the hidden structures behind our favorite fruits and vegetables.

This time, she brought us cheese. Like right now, you are looking at the rind of a milbenkase. There’s a rough sort of graininess to the texture, and you can imagine that it might feel bumpy beneath your fingers.

But you might also notice that some of those bumps are moving. And that feeling under your fingertips is now probably a lot less pleasant to imagine. And that sensation might become worse when you imagine putting some of that cheese into your mouth, knowing that some of those mites will find their way into your body.

Now as gross as it is to imagine, and maybe it isn't. It's just eating animals which lots of people do all the time. I cannot blame the mites, Cheese is delicious!

Except that the mites aren't actually here because the cheese tastes good. It’s really more the reverse: the cheese tastes good to us because the mites are there. Milbenkase is the cheese that it is because of those mites.

It begins as a low-fat curd cheese, kind of like cottage cheese, mixed with salt and herbs, and then it is ripened in a wooden box. Inside of that box are cheese mites, more specifically the species Tyrolichus casei. These mites feed on rye flour that is added to the box, producing digestive juices full of enzymes that help carry out the fermentation needed to develop the cheese.

And after anywhere from a month to a year, you can eat the cheese, mites and all. It’s estimated that there are around 2000 mites in each square centimeter of the rind. Mimolette cheese is also made with the help of these mites, though the species and the processes are a little different.

The mites of mimolette eat the crust of the cheese and leave behind tiny holes that help the cheese breathe. The mite also excretes compounds that impact the ripening of the cheese… which is another not so pleasant image. Despite their differences, milbenkase and mimolette both owe their flavor to the mites.

Scientists have traced the lemony taste of the cheese directly to a compound called neral, which is found in lemon oil… and in cheese mite secretions. Remove the cheese mites from the process, and you take away that flavor. In the case of mimolette, though, the mites are brushed off the cheese before shipment.

And yet enough do remain that sometimes, they can reproduce into a population that is large enough to alarm the FDA. According to news reports in 2013, the FDA didn’t have an official limit on how many mites could be present on cheese. But ideally, they wanted there to be no more than 1 mite per square centimeter.

And the shipment that was making its way into the US had more mites than that, so the FDA stepped in. For us casual cheese eaters, that intervention might seem reasonable. Mites are pests.

We see them again and again in the microcosmos, and they often turn up in places that we do not want them and they cause various conditions that are not comfortable. And in some people who spend a long time working with these cheeses, mites can cause allergies. But for the people who love the cheese, the crackdown on mimolette was not well-received.

This is a cheese that has been around for centuries without much issue. Plus, let’s face it, cheese often includes some strange ingredients, plucked straight from the invisible world of the microcosmos. Like let’s take a moment with this clip.

Just looking at it, where do you think it came from? This is not a trick question or anything. If you saw the blue, and you immediately thought “blue cheese,” well, you are correct.

These samples were taken from the blue veins of the cheese, which are actually dug out by a fungus called Penicillium roqueforti. These molds used to be found naturally in milk and cheese, and people would even take spores from the best cheese of the last batch and inoculate it on bread to use again. But today, spores are often added during the cheese-making process.

As the fungus grows, it breaks down fats in the cheese and produces new flavors The good news is that it is also safe for consumption. As is cheese made with Geotrichum candidum, a fungus used in the production of a number of soft cheeses, where it grows on the surface of the rind. In fact, it is so widely used in French cheese that one estimate suggested that a French person will eat around 8 kilograms of cheese containing Geotrichum candidum in just one year.

Geotrichum candidum lives in a variety of habitats, including soil and grass... but also in raw milk, which is why it’s often found in raw milk cheeses. And with enzymes that break down protein and fat, the fungus can shape the aroma and taste of the cheese overall. In some, it even leave behind a velvety surface to the cheese.

The overall effect of the fungus, though, depends on the strain of Geotrichum candidum. Some create what’s known as “peau de crapaud,” or “toad skin.” And some are known to impart a greasy or bitter quality. And there are times when perhaps those traits are exactly what you want.

In a way, it’s like art made from the microcosmos, a careful adjustment to suit a person’s taste. And maybe that is what made the response to the FDA’s mimolette blockage so strong. On the one hand, it’s cheese.

There are plenty of cheeses available at the store, and to those of us who are perhaps of less discerning taste, it seems like there should be plenty of alternatives should your favorite cheese be held up at the border. But art is art, even if some of the artists are members of the microcosmos. They weave blue veins or construct velvety rinds or add hints of lemony brightness to the flavor, and through centuries of traditions, we have been the wielders and consumers of an art they know purely as survival.

And our own tastes, it seems, have won out. We haven’t found a lot of official information detailing the FDA’s stance on mimolette today. But we did find a 2015 post on a cheese blog that mentions seeing the cheese return to local stores.

Which is good news for everyone… except maybe for the mites that find their way into your stomach. Now we have talked quite a bit about excretions today, including the most notable one of all: poop. But what is poop without a butt to produce it?

Well, that is the question at the heart of our last video today, which is all about face mites. Yes, the mites that live on your face. Because at the center of it all is a surprisingly important question: how many mite butts are on your face?

A lot happened in the year 2022. Economies shifted, reigns ended, wars began. So it is okay if you missed out on this little bit of we think very important news: Demodex have buttholes.

Now perhaps you have never thought to wonder about this. Perhaps you have never even heard of a Demodex before and you’re just slowly putting together that it must be related to the creature that we are watching right now. Yeah, the one that looks like a tardigrade but weirder, which says a lot given that tardigrades already look pretty weird to us.

So you might wonder why we would care if a Demodex has a butthole or not. Well, we care because they live on our face. Demodex are a genus of tiny mites that build their lives around the hair follicles of various animals.

And there are two species that are found on humans: Demodex folliculorum and Demodex brevis. They each have their own preferred nooks on our faces, with Demodex folliculorum preferring hair follicles, and Demodex brevis preferring our sebaceous glands. The mites we are looking at had the distinction of coming straight from the face of our master of microscopes, James, who saw a little black dot on his forehead and was curious what was going on inside.

So he simply scraped the spot with the side of a microscope slide and took a look. But these mites don’t always localize themselves in neat little dots, so sometimes scientists have to turn to other measures to study them. In one NPR interview, a scientist described applying glue to a microscope slide and then sticking it to a person’s forehead so as they peeled the slide off, the mites would peel off too.

And look, we get it. Learning that there are little animals crawling around your face, it might be a little unsettling. Even James, who spends just about every day thinking about microbes, found himself initially repulsed by what he found on his skin.

But then he realized that they look like tardigrades with big butts. And if we can all collectively love tardigrades the way that we seem to, perhaps we can find some space in our hearts for these mites. They’re even cute under the harsh glare of our UV light!

The mites are tiny, less than a millimeter in length. And you can see two distinct halves to their body. At one end—the tardigrade end, as we like to call it in a purely unscientific way—are eight stubby legs waving behind the Demodex’s mouth.

At the other end are their gastrointestinal organs and their genitals. Those spherical droplets in their body are oil droplets, the digested remains of the sebum and moisturizer that Demodex like to eat from our skin. Now, if at this moment, your mind is conjuring up images of the demodex wandering your face in search of oil to eat, you can rest easy.

Well, for now at least. They’re probably not wandering, they're just chilling out inside your pores. At night though, Demodex do like to party.

They emerge from your pores in search of each other, looking for other mites to mate with. And when they are done, they head back to your pores to lay their eggs. An adult Demodex folliculorum lays around 20-24 eggs in your hair follicles.

Within three to four days, the young Demodex will hatch, emerging with only six legs weirdly enough. As they develop into adulthood, the other two legs will develop as well. Their life span will end a few weeks later, at which point the dead mites will decompose in your follicles or sebaceous glands, turning the tiny little pockets of your skin into little mite graveyards.

It's a lot, I know. We keep trying to say that Demodex are cute actually, but then we talk about little mite mating parties and mite graveyards on your skin, and none of that is cute. But here’s the thing: we pretty much all have them.

If you’re watching this video, you probably have Demodex living in your face. And if you don't now, you will eventually. Now they usually spread through physical contact, though a 2015 study found that we tend to primarily pass them on via close contact with our family members rather than strangers.

And when they have found your face, they’ll probably stick around, hiding in your pores and other spaces that make them hard to scrub out. Generally, that’s fine. Demodex don’t wreak much havoc on your skin.

They can become too abundant, creating a distinctive white sheen that dermatologists call Demodectic frost. That excess of mites is usually connected to a decline in the immune system. But these cases are very rare.

For a time, there was a theory that Demodex could cause rosacea, because they were thought to not have a butthole. What do those things have in common with each other? Well the idea was that without an anus, the Demodex couldn’t poop.

Instead, the Demodex would spend their short lives accumulating waste in large cells in their body, and storing it away. And when they died, the feces would burst from their bodies, releasing bacteria that could then cause rosacea. There are enough correlations between Demodex and the occurrence of rosacea to leave us considering that there could be a connection between the two.

But we are here to vindicate the mites a little. Because that mechanism is based on an idea that would turn out to be very wrong, the idea that Demodex don’t have an anus. In 2022, scientists were studying the genes within Demodex.

And among their results are some microscopy images and these words, which we hope will become a classic in the history of butthole science: There have been several reports that Demodex does not have an anus, and when Demodex dies, the accumulated waste spills into the pores of the skin and leads to inflammation; this is not correct. We’ve talked before about just how mysterious and confusing the history of anus evolution is, and so we were excited to hear this news. But the other results of the scientist’s genetic study of the Demodex are also worth learning more about because they reveal just how strange our relationship with these creatures is.

In particular, the scientists found that the Demodex are surprisingly simple, relying on a small number of proteins to survive. This likely drives their nocturnal behavior, because they lack the compounds that allow their relatives to survive at all in the daytime. And this might be the result of just how lonely the Demodex mites are, in an ecosystem built out of our skin where they face few threats and little competition.

They’re doing so well that they may require less than they once did to survive. And that has been pushing them—at least according to these scientists—to a future where they might be symbionts with us, much like bacterial endosymbionts that have become fully embedded in their hosts. That is, for now, just a hypothesis.

If there comes a day when the hypothesis is fulfilled, well what do we do then? Do we consider them weird and creepy, and maybe even a little rude? Or do we accept them as part of us, as inevitable as every other cell in our body?

Or perhaps it is both, as nuanced as any other experience we have with our bodies. Wonderful and terrible, all at once. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

One thing that you've got to know, as a Patreon patron of Journey to the Microcosmos, is you don't know where the next episode is going. But you trust us, to go in the direction that we want to go in to discover what we can discover by not looking big but looking small. The people on the screen right now, they are our Patreon patrons, and they are the reason that we can do this.

If you would like to join them, you can go to Patreon.com/JourneytoMicro. If you want to see more from our master of microscopes, James Weiss, check out Jam and Germs on Instagram. And if you want to see more from us, there's always a subscribe button somewhere nearby.