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These little hairy-bellied friends lead a very interesting life, albeit a short one.

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It takes a long time to make a human.

We can sometimes, I think, get caught up in our own perceptions of how life works. But there are some organisms that don’t have the luxury of years and years of childhood.

No, for them, life is brief and they’ve got to get on with living...and so sometimes like with the gastrotrich, they’ll just go ahead and have their first baby on the day they’re born. Now we’ve never had the chance to name a species of microbe. We name individuals all the time, this one’s Neil.

But naming a species seems like both a thrilling and weighty task. You take this strange, new, tiny thing, and you assign it an identity that it will never know but that other human observers will come to rely on, ostensibly forever. And through the years, people who have faced this challenge have turned to a wealth of imagery and language to guide them.

There’s the multi-headed serpent of Greek and Roman mythology, now also the name for the multi-cellular, regenerating wonder of Hydras. Or you could go cuter, like the tardigrade, which translates roughly to “slow stepper” which also has the advantage of being accurate. And then, you have Gastrotrichs, also known by their phylum name Gastrotricha.

These organisms were not named in so flattering a fashion. No, Gastrostricha translates to “hairy stomachs.” And when you don’t feel like saying “gastrotrich,” you can also call them by their common name: “hairy backs.” But a rose is a rose by any other name, and, truth is, they do have hairy stomachs. And backs.

All, as we will see, for good reason. But if their name doesn’t capture the mythological glory that other organisms get, there are moments, when the light hits just right and bounces off their little scales, that Gastrotrichs resemble iterations of one of the most pervasive mythological figures of all time: the dragon. We should scale back a bit though.

After all, gastrotrichs are among some of the smallest metazoans, or we could say, less esoterically...animals. They range from around 60-500 micrometers long, though in some very exceptional cases, they might get up to several millimeters in length. There are two orders in the Gastrotricha phylum: Chaetonida, which are mostly found in freshwater, and Macrodasyida, which are found in marine systems, though there are, weirdly, a few freshwater species.

No matter what order they belong to, they make up part of what’s called the meiofauna. These are species grouped by their size. Too big to be microfauna, too small to be macrofauna, the meiofaunal organisms are between 60 micrometers to 2 millimeters—and they play a deeply important ecological role, a bridge between the micro and the macro, consuming the former and being consumed by the latter.

They are a part of this system that brings nutrients out from the microscopic world, into the parts of ecosystems that we can directly observe. Gastrotrichs are among the most abundant species present in the meiofauna. In some habitats, it’s been estimated that there are around 100,000 individual gastrotrichs per square meter.

And there are around 860 known species counted across both orders. Now, that’s a lot of species of Gastrotrichs, but it’s actually surprisingly low compared to the diversity of other meiofaunal organisms. There are, for example, over 2,000 rotifer species, and approximately 40,000 species of nematode.

Now this doesn’t mean anything bad for gastrotrichs—though it might be why they have yet to be found in any extreme kind of habitat. But it is remarkable that for microscopic metazoans, 860 species is low when you put it up against some other phyla. Returning to their unfortunate but well-earned name: the stomachs of gastrotrichs are covered in hair-like cilia, which they use to glide along surfaces as they dine on bacteria, algae, and other small particles.

Unfortunately for us, it is hard to observe and film these hairy stomachs because, well, they swim with their bellies down. We’ve found that when we record gastrotrichs, we have to be extra careful. Their head is covered in cilia and mechanoreceptors that seem to respond to any slight movement in the water.

So it’s important not to create any accidental vibrations in the microscope that might startle them. Gastrotrich bodies are enclosed in a cuticle and covered in spines or scales, or sometimes both. And at the end of their bodies, many species have a pair of feet, which hold adhesive tubes that they can use to attach to grains of sand or other objects.

And underneath the hairy, scaly cuticle, we see the wonder of a multicellular body: a brain, nervous system, simple gut, intestines, muscles, and more. Inside of this chaetonid gastrotrich is an egg, and when that egg hatches, out will come a new gastrotrich, fully developed with all of those organs in place, and about 2/3 the length of a full adult. The fact that it hatches so formed is remarkable on its own, but the whole reproductive cycle of chaetonids is a marvel.

Gastrotrichs are not particularly long-living creatures. Their life span is on the order of days to weeks, which means they have to cram a lot of living into a short amount of time. When they hatch, chaetonids seem to immediately begin developing their eggs through parthenogenesis, a kind of cloning through which an unfertilized egg is able to develop into an embryo that is genetically the same as the parent.

In the species and conditions that chaetonid reproduction has been most extensively documented in, you might see a gastrotrich lay an egg within a day after they themselves hatch. And typically, they’ll lay 4 eggs within 4 days that can develop and hatch within a day of being laid, producing genetically identical offspring. And those children are then out there themselves laying eggs, meaning a gastrotrich can become a grandmother just a few days after it was born!

And here’s another weird thing...sometimes, they lay a special kind of egg called an opsiblastic egg, which has spines and is thicker and larger compared to the other eggs, qualities that keep it protected under harsher conditions. Because of this rapid egg laying, we used to think that chaetonids were entirely made up of parthenogenetic females. But later observations have revealed that once these early eggs get laid, the chaetonids become hermaphroditic, developing sperm and eggs.

What they use the sperm for though is a mystery. We just don’t have the observations we need to understand how chaetonids make use of their hermaphroditic phase. This life history though characterizes just the chaetonids.

Macrodasyoid gastrotrichs seem to all be hermaphroditic except for one species, and so their process is of course different. And yet the result is the same: the direct development of young gastrotrichs that contain the full multicellular plan in their own body, ready to create the next generation at birth. It’s hard to know sometimes what to be most impressed by when you’re looking into the microcosmos.

Is it the giant unicellular organism, or the tiny multicellular one? Is it giving birth on the day you were born, or the 40,000 species of nematodes..of course, chances are, just minutes ago you had no idea that the earth was covered in these tiny beasts, bridging the words of the micro and macro, and now we can’t even decide what the coolest thing about them is. Complexity is its own complex thing, so carry on, our hairy-bellied friend.

And thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you especially to all of the people on the screen right now who support us via Patreon. These people allow us to continue diving into these mysteries.

It’s a joy to do and we’re so happy that you are compelled to help us out. If you want to see more from our Master of Microscopes, James, you can check him out on Jam and Germs. And if you want to see more from us, here at Journey to the Microcosmos, you can find us on twitter at and you can also find us here on YouTube at