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We know these cute little water bears can survive the vacuum of space but are they actually immortal? We'll explore that and other misconceptions about tardigrades in this week's journey!

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Now, we didn’t want to start this channel out with a video about tardigrades.

We wanted to share at least a bit about some of the other mind blowing organisms of the microcosmos first. But we also knew it wasn’t going to be long before we got to them.

We are, it must be admitted, tardigrade enthusiasts. And we’ve been excited to watch as a fair number of people have joined us in that enthusiasm. However, of course, and we’re not mad about this, the popular narratives about tardigrades have, in their enthusiasm, gotten a few things less than correct, and also skipped over some of the most interesting facets of this celebrity of the microcosmos.

So...tardigrades. They’re chubby and they have heads and legs and butts and they look, at first glance, somewhat familiar. That’s why we sometimes call them water bears or, even better in our opinion, moss piglets because they are often in the aquatic environments that cling to mosses.

But they are also very unlike macro-animals. For one thing, they have eight legs. For another thing they are not closely related to anything else on earth.

Though taxonomists are still fighting about this, the leading system places tardigrades inside a clade called Tactopoda that includes two groups. There's tardigrades and there's something called. Euarthropoda.

And that other clade includes basically every bug, insects, arachnids, even crustaceans and trilobites. The organism that branched away from tardigrades has literally millions of species as its ancestor. But the branch that tardigrades sit on...just tardigrades.

Though there are over 1000 described tardigrade species and there are likely many times more than that out there. But nonetheless, they do look a lot like bears with their claws and stumpy four pairs of legs. Tardigrades hover right on the edge of microscopic.

They range widely in size from 50 microns long to 1200 microns but usually, they're under 500 microns in size. Tardigrades can be found in three major aquatic environments. Some live in marine or brackish water, others in fresh waters like ponds, lakes, and rivers, and then there are the terrestrial habitats but this isn't exactly what it sounds like.

See, tardigrades do live on what we would consider land, but they live in films of water clinging to plants and dirt. This thin film, though nearly invisible to us, can be a Tardigrade’s whole world. But these films can easily dry up in the daytime...which is part of why Tardigrades are so good at surviving difficult situations, which of course we’ll be discussing shortly.

They occupy almost every place on earth, from the Arctic to the Antarctic, from soil to beach sand, rivers, lakes, streams, on mosses, lichens, algae and plants. We know of a couple of places where we will almost certainly find them waddling around in samples we take. And we will make a whole episode about that process but that’s for later.

Although they are aquatic, tardigrades don’t swim, they kind of walk. In fresh water, they live among vegetation and mainly feed on the cell contents of plants and algae by piercing the cells with the two stylets they have in their mouthpart but some species eat single-celled organisms or even other micro animals including rotifers and also other tardigrades. Some species of tardigrades have eyes, besides helping them look super cute, they also boggle the mind because they aren’t eyes.

To be an eye, you have to be an organ, and to be an organ, you have to be composed of tissues that are themselves composed of cells. But each tardigrade eye is, in fact, a single photoreceptive cell. That is wild.

On their whole body, sometimes containing tens of thousands of cells, they have two cells, and those two cells, are there two little eyes. Tardigrades are also covered in a kind of skin, or shell, called a cuticle. Coloration is produced by the pigmentation of the cuticle and also the content of the digestive tract.

Gray, bluish, yellow brown, reddish or brown are common colors. This cuticle always stays the same size, and so must be shed as the tardigrade grows. Some species of tardigrades lay their eggs into this cuticle, which expands to hold them.

And then when the cuticle is shed, the eggs stay inside, giving them an extra layer of protection from roving predators. If those eggs manage to avoid being eaten, they will hatch into tiny, little tardigradelettes that, oddly enough, have the same number of cells that they will always have. But their cells, and thus their bodies, will grow larger.

Tardigrades are marvelously complex for such tiny animals, they have digestive systems and salivary glands, even small brains. The most complex have over 40,000 cells. And yes, when they’re done extracting nutrients from their food, they have to get rid of it just like the rest of us.

Of course, tardigrades are best known for their survival skills. Many species have the ability to survive environmental extremes, such as temperatures as low as -272 degrees Celsius and as high as 150 degrees Celsius. Extreme radiation.

Extreme pressures as high as 6000 atmospheres. NASA sent a bunch of tardigrades to outer space and brought them back alive after exposing the animals to the vacuum of space and extreme radiation from the sun. A tardigrade may have a life-span of a couple of weeks to more than 100 years.

A museum specimen of a dried moss that was in a herbarium for 120 years yielded active tardigrades when soaked in water. Tardigrades are able to survive these extremely hostile conditions by undergoing a process known as cryptobiosis. In cryptobiosis, the metabolism of the tardigrades can lower to less than 0.01% of normal and when conditions get better the tardigrades simply get back to life.

There are actually several of kinds of cryptobiosis. In Anhydrobiosis are dealing with a lack of water, and they curl up into a ball called a tun to being all dried up. A glass-like protective sugar, trehalose is synthesized and it replaces the water in the cells of the tardigrades.

This sugar prevents crucial parts of the cells from getting damaged during water loss and lets tardigrades to survive the dry period. Anoxibiosis occurs when there is insufficient oxygen. The body of the tardigrade swells and becomes rigid and turgid.

There is no movement and the tardigrade looks dead. This sometimes happens in our prepared slides. So when it happens, we simply blow air into the slide from the side for around 10 seconds and that is enough to increase the oxygen concentration and “revive” the water bear.

And there are some other types of cryptobiosis such as cryobiosis where tardigrades can withstand extremely low temperatures and osmobiosis where they can withstand changes in osmotic pressure. One of the misconceptions about tardigrades is that they are immortal. They are resistant to these extreme conditions when they are in tun state, but otherwise they are quite delicate animals and can be hurt and killed easily.

Because of their small size, tardigrades are eaten by some eve bigger single-celled organisms. In this video, we managed to record a close encounter between tardigrade and a giant eukaryote. We don’t even know what this single-celled organism here is, let us know if you think you can identify it, but it could easily eat the tardigrade and digest it.

But this lucky little one managed to escape and even damaged the cell of the eukaryote in the process. And in this clip we have a happy tardigrade waddling on one of the most dangerous residents of the ponds, a brown hydra. Hydra’s tentacles are armed with something called nematocysts.

These are like tiny harpoons that are fired upon contact to immobilize the hydra's prey. This water bear was either very lucky or too small to activate the nematocysts and so got away from a heartbreaking end. Tardigrades are not extremophiles, they don’t enjoy living in extreme conditions they are just good at surviving them.

In that way, maybe they are something like us. We also sometimes live through situations we thought would be impossible. But, all things being equal, both tardigrades and us just want a nice place to live, plenty of yummy food, and whenever possible, to not be exposed to the vacuum of space.

We are so happy that these friends share their world with us, so often just outside of our notice, but successfully chomping their way through the microcosmos for more than half a billion years. And just in case you wanted to see it again... (laughs). That's good.

Thank you for coming on this journey with us through the unseen world that surrounds us. If you want to see more from our Master of Microscopes, James check out Jam and Germs on Instagram. And if you want to see more from us here at Journey to the Microcosmos,.

I bet there's a subscribe button somewhere nearby. We really appreciate everybody's kind words in the comments. It really is just a joy to make this channel.