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You might climb up a tree to get a bird’s eye view of the world, or to pick some fruit.
But for our master of microscopes, James, trees are a way to get a very close up view
of something much smaller than an apple.
Yeah, it’s time to talk about tardigrades.
Along with all the squirrels and birds and bugs scurrying up and down and around trees,
there are also tiny little tree tardigrades tucked into the watery drops inside moss and
lichen that grow on the tree’s surface.
They’re not actually called tree tardigrades, they’re called terrestrial tardigrades,
because they live on land and they’re a little bit different from the tardigrades
we’ve shown here thus far.
Those have been from freshwater sources like ponds and rivers, and they’re most likely
of the genus Pseudobiotus.
And why do we feature them so much?
Well, it’s certainly not because they’re easier to find.
In fact, we have a harder time tracking down the freshwater species than the terrestrial
ones.
James knows exactly where to find terrestrial tardigrades: his front yard.
They live in some of the old trees there, and when it rains, James will scratch some
patches of moss off them and then bring them inside to observe the tardigrades hidden in
their depths.
So why the freshwater bias?
We’ve got two answers for you, one is scientific, one .... is not.
On the science side, terrestrial tardigrades may be easier for us to find and gather, but
moss samples can be tricky to work with.
They contain lots of remains from other animals that live in trees along with tons of bacteria,
which divide really fast and consume all the oxygen on the slide, suffocating other life
like our water bears.
We don’t have this same challenge with their freshwater counterparts, so it’s easy for
us to keep them alive in our slides to watch for months.
And that is why we’ve been able to document everything from their mating habits to their
hatching eggs to their food—it’s like our own little tardigrade reality TV show.
But we also said there was a less scientific explanation for our Pseudobiotus bias.
And we don’t want to hurt any feelings here, but really it’s just this: We like them.
They’re just very cute.
They’ve got those wiggly legs and the beautiful black eyes, and somehow they just stand out
even from other tardigrades.
But we need to set aside both these scientific and superficial excuses because terrestrial
tardigrades are pretty cool.
The ones we find usually belong to one of two genera, both of which you can see here.
That reddish one is in the genus Ramazottius.
And the other one is Milnesium.
The name of the Ramazottius genus comes from Giuseppe Ramazzotti, who was a tardigradologist,
which—yes—is the actual name of an actual job.
And their reddish color comes from pigments in their cuticle, which is, basically, their
exoskeleton.
We think this color might be a way to protect the inside of the tardigrade from the sun’s
radiation...basically, tardigrade sunscreen!
Whether or not that is true, we can certainly appreciate how the colors make it look like
a ruddy, half-translucent alligator with way too many legs.
While we’ve talked about tardigrades quite a bit on this channel, one of their weird
quirks that we haven’t gotten into are their coelomocytes.
See, tardigrades are multicellular organisms, but they don’t have a circulatory or respiratory
system.
So, how do they deliver all of the things they need to deliver to all of their cells?
Well, they have coelomocytes.
These are little storage cells that hold sugars and proteins and lipids, and they transport
them around the tardigrade’s body.
It’s like little presents delivered from cell to cell in a bubble that floats around
the organism.
It’s kind of adorable.
In some tardigrades, the number of storage cells stays constant.
But in others, like our Milnesium tardigrades, the number of storage cells will go down when
the tardigrade is dealing with starvation, or when they go through anhydrobiosis, that
famous state where the tardigrades dry themselves out to survive.
The milnesium tardigrade may not have the color of the Ramazottius, but it is still
very fun to watch.
Here you can see its little legs at work as the tardigrade ambles across the slide.
It’s like watching your cat’s paws from underneath a glass table, but instead of toe
beans, you got micro claws.
And those claws are one of the morphological characteristics that help distinguish between
different species of Milnesium.
Indeed, we can even see these claws in an extinct ancient species, Milnesium swolenskyi
And yeah, it is weird that we know about an ancient tardigrade species and that we can
tell it’s in the milnesium genus from it’s little claws.
How?
Well, someone, and thank goodness they did, looked very closely at an 89.3 million year
old glob of amber dug up in New Jersey and found, inside, the only known example of milnesium
swolensky...complete with tiny microclaws.
Milnesium tardigrades are carnivorous...even their larvae are able to feed on rotifers
soon after hatching—though they usually aren’t able to eat a whole one until they
grow a bit.
Eventually, as they mature, they’re able to swallow a rotifer whole.
As we mentioned before, terrestrial tardigrades go through anhydrobiosis, which translates
to “life without water”.
And this is not just a quirk of tardigrade existence.
This is how they survive on land.
Life in moss is a life of extremes.
When there’s water, things are great.
But when the moss dries out, which it does, the tardigrades inside it must somehow survive.
With no water surrounding them, their bodies, when this happens, can drop to 1-3% of the
water content it had before.
And that tardigrade will shrink, like this one did, and turn into what’s called a tun.
It will remain metabolically at rest, famously capable of withstanding all sorts of extremes,
until water returns to its surroundings and fills the tardigrade up with life again.
The main thing we know about tardigrades is that they enter this state which allows them
to survive seemingly indefinitely and deal with all kinds of extreme environments.
And how these tardigrades enter and survive the tun state is complex and mysterious, and
different species are able to withstand more extremes than others.
Another thing I don’t think we talk enough about when talking about tardigrades, is that
there are likely upwards of 1000 species of tardigrade!
For comparison, there are 6500 mammals...total.
ALL MAMMALS.
6,500.
Tardigrades, over 1,000 all by themselves.
So, you gotta wonder, in the face of all of these tardigrade species, how do scientists
even pick which ones to study?
Well, much like how we’ve got our reasons for the tardigrades we focus on here, they
have theirs.
One group of scientists picked Ramazzottius varieornatus because it’s one of the most
stress-tolerant species, an extreme of extremes that helped the researchers find a tardigrade-unique
protein that protects their DNA from damage.
And sometimes, it’s just a matter of logistics.
Astrobiologists turn to extremophiles like tardigrades because they might give us clues
into what multicellular life in extreme environments might look like.
But Earthly scientists are still subject to Earthly constraints, like time and other projects
and life away from experiments.
And so sometimes, logistics are the most important thing, which is why one group put forth the
argument that while the Milnesium tardigrade might have a lot of the wondrous survival
capabilities they want to study, they’re also just a pain to take care of.
Their carnivorous diets mean they need to be constantly fed, and they just need a lot
of care.
But—the scientists pointed out—the Ramazzottius varieornatus can survive just on algae and
requires a lot less immediate care, making them much easier to manage as a research organism.
Some might call this laziness, we call it good planning.
You don’t want your entire experiment ruined because somebody can’t come into the lab
to toss a bunch of rotifers in the Milnesium tank.
Every journey comes down to choices, but we’re fortunate that any choice we make with the
microcosmos always seems to lead somewhere fascinating.
Thank you for coming on this journey with us as we explore the unseen world that surrounds
us.
Here’s a quick German lesson for you.
Did you know that the original name given to tardigrades in 1773 was “kleiner Wasserbär”,
which translates to “little water bear”?
If you're interested in learning more German, well, I am not the one to teach it to you
because I just had one cool tardigrade fact ready to go and that’s all I got, but the
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