One with eight legs, and a reputation for invincibility.

Supposedly, they're everywhere.

They've been found on the tops of mountains, at the bottom of the ocean, in tropical rainforests, Antarctica, and maybe on the top of my moldy garage.

But tardigrades are more than just charming and ubiquitous.

They are really intense.

If you've ever heard about tardigrades, you've probably heard that they're invincible.

And there is some surprising truth to this.

Certain species can survive being frozen to just above absolute zero, and heated to more than 149 degrees Celsius.

They can survive being completely dried out, and have even survived going to space.

In these extreme conditions, tardigrades go into a sort of suspended animation.

And even after decades of being in this suspended state, they can be woken up as if nothing happened.

It's so unlike anything we see in our day-to-day lives, that it makes scientists question the boundaries of what we consider alive versus dead.

And beyond this perplexing resilience, even the basic anatomy of tardigrades comes with its own puzzles.

Most creatures in the microscopic world wriggle, thrash, or have beating cilia or flagella.

Legs are almost unheard of for creatures this small.

And the mystery of tardigrade anatomy only deepens when you learn about marine tardigrades that have insane structures coming off their body that are as dazzling as they are baffling.

Some look like flagpoles, others look like inflatable sticky feathers.

And for many of these structures, we still don't know what they're for.

For an organism so prevalent in nearly every part of the planet, tardigrades are still deeply mysterious.

Why did this microscopic creature evolve to walk at all?

And what's with those crazy morphological structures?

Why are tardigrades thought to be invincible?

And could their incredible abilities one day allow them to colonize outer space and the harsh environment of Mars?

All tardigrades are aquatic animals, requiring a film of water around their bodies to permit locomotion and gas exchange.

However, most of the tardigrades known to science are found on land, in mosses and lichens all over terrestrial environments.

These tardigrades are known as limnoterrestrial, and are classified as eutardigrades.

But you can't see them with the naked eye.

They're between 100 and 500 microns in length, and only visible under a microscope.

But luckily for me, it doesn't have to be a very powerful one.

So having recently moved to a very wet, mossy environment, I'm going to see if I can find one.

So the scientists told me that you can find tardigrades in mosses and lichens.

So I'm going to collect some moss from this rock that I just found.

Betty's going to help me.

You helping?

And I'm going to put it in this petri dish, and later we'll try to decant some tardigrades out of it.

The origins of tardigrades appear to go back all the way to the Cambrian, around 550 million years ago, before land plants, before the earliest ancestors of mammals, before the dinosaurs.

Basically, tardigrades have been around for a long, long time.

Today, scientists struggle to fit the tardigrade neatly into the evolutionary tree.

They're in their own phylum, called tardigrada, and there is still much debate about whether they're more closely related to arthropods or to nematodes.

PhD student Mark Mapalo considers himself a paleotardigrodologist and has special interest in studying ancient tardigrades.

The oldest fossil tardigrades that we are sure is a tardigrade is actually around the Cretaceous.

It's dated around the Cretaceous, which is like the time of the dinosaurs, basically.

It's 90 million years old, and all of those tardigrade fossils are actually preserved in amber.

So our hypothesis is that these tardigrades are so small that the only way for you to actually preserve them, or the highest chance of them being preserved in a fossil, is in amber.

Amber is fossilized tree resin.

You're probably familiar with the sticky resin that comes out of some trees when their bark is injured.

Now and in the past, insects or tardigrades often get trapped in the substance and can't escape.

On occasion, some of these ancient globs of resin fell in water and ended up becoming buried in the sediment.

As it settled deeper and deeper into the earth, the pressure and temperature began to rise.

Over millennia, these conditions caused the resin's compounds to polymerize, where they turned hard and glassy.

Whatever organisms were trapped inside the resin, insects, plants, or tardigrades can be preserved with extraordinary fidelity.

Unfortunately, the DNA of ancient tardigrades isn't preserved because DNA is fragile.

The very oldest traces of it we've ever found are only 2 million years old.

But much of tardigrade anatomy is preserved in the amber.

And even though the oldest tardigrade fossil is 90 million years old, it still very much looks like a modern tardigrade.

You would see it and it's actually like still look the same as a living tardigrade.

It's just like amazing how basically their external morphology did not really change that long.

And that's because the tardigrade body plan is remarkably effective.

At the head, they have mouth-like organs equipped with piercing stylets.

They use these freaky little tongues to pierce the cell walls of plants, algae, and fungus.

Their esophagus then sucks the food in and the nutrients spread from their digestive tract to the rest of the body.

But some tardigrades eat more than just plants and fungus.

Some are predators that consume entire living organisms such as rotifers, nematodes, and even other tardigrades.

Predatory tardigrades can even have a significant impact on the biodiversity of other micro-animals around them.

Scientists found that hungry tardigrades will eat up to 56 nematodes a day in certain conditions.

And this can be really beneficial to the soil quality if those nematodes are pests who parasitize plants.

Also on the first body segment, some species of tardigrade also have very basic eyes made of just a handful of visual cells that allow them to detect light.

But like with everything else with tardigrades, we're not exactly sure that's the whole story.

Recently, scientists identified multiple R opsins in tardigrades that were associated with vision.

But these opsins didn't seem to help them with color vision as they might in other animals.

Weirdly, they weren't even really active in the adult tardigrades.

These opsins were most active when the tardigrades were still eggs.

What on earth eggs need visual opsins for, no one has any idea.

Next on the tardigrade's body are the three trunk segments, all of which have a pair of legs on either side and claws on those legs.

And the fifth and final segment of the body has a pair of legs that face backwards.

It's thought that the orientation of these legs help the tardigrades grip onto things, almost like a prehensile tail.

And the existence of all of these legs make tardigrades really strange.

Microscopic animals the size of tardigrades rarely have legs, and if they do, they aren't used for walking.

Water fleas, for example, have legs, but they're used for swimming and sweeping food into their mouths.

Rotifers of similar size swim or inchworm along their substrate, and roundworms sort of just wiggle around.

Walking in this microscopic domain is kind of unheard of.

One reason it might be so rare is because walking in water while being so small requires overcoming a ton of viscous forces.

It would be like walking through honey for us.

Tardigrades also have to overcome an extremely variable environment, moving through syrupy water, climbing over piles of sediment, or through clumps of tangly plant matter.

Yet tardigrades use their eight stubby legs to walk through all of this with ease, the world over.

Does the smallest-legged animal have some unique method of using their legs to overcome these obstacles?

To find out, scientists looked at the gait of tardigrades with specialized cameras.

They found that when tardigrades walk slowly, they lift one foot at a time.

As they speed up, they lift two feet that are diagonal from each other across the body, keeping four feet on the ground.

And as they go even faster, three feet are off the ground at once.

They keep a minimum of three feet on the ground at all times, even at their fastest speeds, not including the backward-facing legs.

This differs from many fast vertebrate gaits, like a horse's gallop, where all four feet come off the ground at once.

While the tardigrade walking pattern may seem random to us, it's actually not unique at all.

The scientists realized that despite having significant differences in size and skeletal structures, this way of walking was very similar to insects, like larger panarthropods such as stick insects, insects about 500,000 times their size and separated by about 20 million years of evolution.

But what's the benefit of this type of walking?

For stick insects, always keeping three or four feet on the ground provides them great stability over pointy, jagged, and variable twigs and branches.

For tardigrades, keeping three or four feet on the ground may similarly help provide stability as they trudge through variable, complex terrain.

So tardigrades can't run fast like a horse, or a micro-horse, I guess.

So they're slow, but most importantly, they're steady.

This similarity between tardigrade walking and insect walking was a surprising result, and one that may point to the reason behind the existence of tardigrade legs in the first place.

It could be that arthropods and tardigrades share a common ancestor that had legs much like this, neurally wired to walk with this pattern.

Thus, this could be a piece in the puzzle of tardigrade taxonomy, putting them closer to arthropods than nematodes after all.

And further assisting tardigrades in their journey through thick and tangly environments are the claws at the end of their legs.

Many tardigrades actually have complex double claws on each leg, which consist of two slender primary branches and two basal secondary branches, which themselves have two or three hooks.

These hooks help the tardigrade hold onto the substrate so they don't get carried away.

The difference in the number of claws and their shape is an important way that scientists can distinguish between different species.

And there's something else fascinating about all the cells that make up those claws, legs, and body segments of a tardigrade.

Every cell that makes up every one of these body parts is a cell that the tardigrade has always had.

Tardigrades don't grow by cell division like we do.

Their growth occurs by enlargement of the individual cells rather than by cell division.

And adult tardigrades of the same species will all roughly have the same number of cells, some with up to 40,000.

This phenomenon is known as eutely.

It's possible that growth like this decreases the risk of cancer or other problematic mutations that arise during cell division.

With all of this incredible anatomy combined, tardigrades live basically everywhere.

In freshwater lakes, rivers and ponds, and on every single continent.

So if I were to get a microscope, what do you think my chances would be if I went out into the world to look for tardigrades?

Where are you right now if you don't mind?

I am in Connecticut, in New England.

Okay, yeah.

Well, I would say there's a high chance of you finding a tardigrade there.

I feel like I always find tardigrades in New England.

I don't see… It's kind of tardigrade-shaped.

Oh, there's something moving.

Oh, what was that?

But as much as I'm struggling to find a terrestrial tardigrade, there's another type that's even harder to find.

The marine tardigrades.

Terrestrial tardigrades may be amazing, but marine tardigrades are like something out of a Dr. Seuss hallucination.

Marine tardigrades are usually heterotardigrades, and they can be divided into three major ecological groups.

There's the species that live on the slime of algae or the plates of barnacles, and are sometimes known as ectoparasites.

There's the interstitial species, which can be found in the top few centimeters of and there are the deep-sea benthic species.

Marine tardigrades are characterized by their cephalic sensory structures that are absent in eutardigrades, and many of these structures are downright flamboyant.

You know how when you look at a, if you look in a stream and you look at macroinvertebrates in a stream, they're cool, but then you look on a coral reef and it's like, whoa, mind-blowing?

Tardigrades are like that.

Terrestrial and freshwater tardigrades are cool.

You look at marine ones and they're mind-blowing.

Dr. Paul Bartels has been studying tardigrades since 2000, and more recently started focusing on marine tardigrades, and I can totally see the appeal.

Some of these tardigrades are truly spectacular, like Tenarctus bubalubus, which was found in the Atlantic around the Faroe Islands.

It's got these posterior branches that come off the back end.

They branch and branch and branch and branch, and then at each end there's these bubble-like formations that can inflate or deflate, and they can, when they deflate, they're like adhesive, and when they inflate, they can be buoyant.

And so it's this like Dr. Seuss-like character with these big balloons coming up the rear end, and it's just crazy stuff like that.

And that's hardly the only odd-looking heterotardigrade from the ocean.

Some have adhesive claws.

Some have wing-like structures that are extensions of the cuticle.

Some have long, inflatable tails.

One of the ones I'm really interested in is there's an intertidal one that's called Batilipes bullacaudatus.

Batilipes are these ones with little suction cup toes, and they live mostly in intertidal sands.

The suction cups allow them to hold on to shifty, intertidal beach sand.

But they vary.

There's about 40 species.

It's the most speciose of all the marine tardigrades, and one thing that they clearly vary in from species to species is their caudal structures.

One of them that one of my students found near her home in the Outer Banks of North Carolina has this tail that is like, it's like a flagpole at the end of the body.

It sticks up almost vertically from their rear, and at the end of the tail, there's this big membranous bubble, like a balloon.

But what are these ridiculous structures for?

For some things, scientists have a decent idea.

Some of them are kind of thick paddle-like structures, and some of them are more filamentous hair-like structures.

And we know that at least on some of the paddle-shaped ones, they have a pore at the end.

So presumably, I think there's pretty good evidence that those are chemosensory, and the hair-like ones are probably tactile.

And for the crazy-looking caudal structures like the flagpole?

For things like this, there's less certainty.

One theory is that it helps them hold on tighter to their substrate, like an anchor.

But the flagpole isn't mucusy and sticky like the feathers of Ternarctus bubulubus, so it might not actually stick very good.

It could be that it gives them some buoyancy.

They don't, marine tardigrades don't have any larval planktonic stage like almost all macroscopic marine animals do.