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If we had to nominate an ambassador to represent the microcosmos, we would have to go with the tardigrade. They’re weird, adorable, and hardy, – a combination of traits that has made them many people’s first entry point into the microcosmos.

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If you don't know, Journey to the Microcosmos has its own Microcosmos Microscope.

A microscope we designed specifically for people who are new to the hobby of Microscopy. Who maybe want to discover their very first tardigrade.

It's a really wonderful moment, I had mine about two and a half years ago. And with the Microcosmos Microscope you are able to attach your phone to it So you can take pictures and video of the things that you discover. When I first wanted to try out microscopy, I found the step of finding the right microscope to start with very intimidating, so we're just trying to make that easier for everybody.

You can find it and a bunch of other stuff like great shirts and socks to celebrate the microscopic world around us at 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 desiccated, 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 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. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

Big shout out to the people on the screen right now. They are our Patreon patrons. They’re folks who think that there should be a YouTube channel that, in a calm and serene way, discusses the beautiful splendor of the mystery of all the tiny things that are all around us all the time, that for the vast majority of human history, we had no idea were even there.

So if you're happy that a channel like ours exists, it's people like these who make that possible. If you want to see more from our master of microscopes, James Weiss, you can check out Jam and Germs on Instagram. And if you want to see more from us, there's always a subscribe button somewhere nearby.