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We also don't really know what rotifers are... but we'll try to tell you as much as we know!

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What is this thing?

In fairness, when the first microscopes were turned on samples of muddy water, that was a common question. And the discoverers of that time were seeing new things every day that puzzled and baffled them.

But you would think that, by now, we’d have a pretty good idea. And yes, this is a rotifer...but...what is it? There are around 2,000 species of rotifers distributed in watery habitats around the world.

And so it’s no surprise that they’ve come up on our channel quite frequently. Unfortunately, their situations on here are usually quite dire. They’ve been captured.

They’ve been swallowed alive. Sometimes their body fluids are getting sucked out of them. You may even remember the one rotifer trying to lay its egg as it was slowly being eaten by a heliozoan.

So we need to cut the poor little rotifer some slack, maybe even make a case for them. Even if, or maybe because, they seem to have a hard time doing that on their own. Even if we, even now, don’t know precisely what they even are.

Rotifers are definitely an animal. And they are a type of invertebrate, but beyond that, here we are are in 2019 and, like, we just really don’t know. They’re just really...different from other animals, though we think they might be related to arrow worms, a tiny animal that is a major component of marine plankton, that’s something we first uncovered literally last year.

It’s been challenging to map out the evolution and relationships of the Rotifera phylum because despite their apparent propensity for dying, their microscopic, soft bodies make for poor fossils. We homo sapiens have only found one single rotifer fossil from outside of the Holocene, which is our current geological epoch. We have one ONE sample of a rotifer...fossilized in Amber from around 35 million years ago, during the Eocene, and it looks weirdly identical to rotifers that still exist today.

The vast bulk of the Rotifera phylum, make their home in fresh water, and they can be found around the world, from small puddles, to sewage ponds, to crisp, Alpine lakes. They may be microscopic, their bodies typically ranging from 100 to 500 microns long, but their tiny bodies contain even tinier organs: from a simple brain, a small stomach a kind of jaw, a nervous system, all compartmentalized in various regions of its body. Rotifers come in many different shapes, but you can generally break their bodies down into four regions.

You’ve got the head, the neck, the body, and the foot. At the top of the rotifer’s head is the corona, a crown made up of cilia whose movement resembles a turning wheel and that is what gives the organism its name: in Latin, “rotifer” translates to “wheel-bearer.” The corona helps the rotifer eat, the beating of the cilia creating a vortex that draws food, like micro algae, into the mouth. Then the rotifer uses its jaw, called the trophi, to grind down the food.

The hardness of the trophi makes it one of the few parts of rotifers that we can find fossils of, and along with the corona, it is one of the unifying characteristics of rotifers. Some rotifers reproduce strictly through parthenogenesis, a form of asexual reproduction, while others use both asexual and sexual reproduction. Either way, rotifers are eutelic, which means that from the moment they are born, they will have a fixed number of cells in their bodies for the rest of their life.

For most rotifers, that number is around 1000 cells . And for the rotifer to grow, the cells themselves—which cannot divide—have to get bigger. About a quarter of the cells in a rotifer’s body are dedicated to its nervous system, which lets the rotifer process its surroundings and respond accordingly.

As you may have noticed, a lot of rotifer species are quite stretchy, extending and contracting as the organism navigates the world. At the end of the rotifer’s body, where the foot lies, you might find glands that secrete a sort of glue that the rotifer can use to attach itself to a surface. Now, some rotifers form colonies, which can be as small as two members, to some much larger colonies that have more than 1,000.

These rotifer gatherings can form through different recruitment methods. Sometimes a colony is already established, and a young rotifer comes by and joins in. Other colonies are constantly developing and expanding as young rotifers are born into them, getting smaller only as the colony dies or breaks up.

And sometimes, a bunch of young rotifers decide to just trek out and establish their own colony together. How do they decide to do that? I dunno.

They’re mysterious. For all these different ways they start, rotifers inside a colony don’t seem to share resources, so it’s not quite clear what the colony’s purpose is. Maybe it helps protect them from predators.

Or maybe it’s….something else...remember...mysterious. While rotifers can live in the thin films of water found in more earthly environments, their habitats can change. Moss, lichen, or soil, where rotifers may take up residence because of the dampness, are still subject to the daily and seasonal water cycles, and as evaporation dries out their surroundings, many rotifers protect themselves by entering a sort of resting state called anhydrobiosis.

We don’t have an image of this to show you, but imagine this rotifer shedding water from its body until it’s about 30-40% of its original weight and compacted into a little ball, appearing from the outside like nothing more than a tiny grain of silt. But when water comes back to the rotifer, it will switch out of this state and back to an active one, waking up without having aged like a microscopic Sleeping Beauty. Even though they are microscopic and often ignored by us macro-animals, and apparently often bullied and consumed by their single-celled neighbors, rotifers are charmingly bizarre animals.

They are everywhere, minding their own business, living their own lives, and turning themselves into inactive little balls when life gets tough, which makes them, you know, not unlike us sometimes. So let’s end with a tribute to the rotifer, giving it the dignity of words spoken to the. Royal Society in the 18th century by rotifer-observer, Mr.

Baker: “I call it a Water Animal, because its appearance as a living creature is only in that Element. I give it also for Distinction Sake the Name of Wheeler, Wheel Insect, or Animal;”. Wheel away little friend, whatever your lineage, however you came to exist. Wheel away.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. Thank you, especially, to all of our patrons. You can find us at And if you want to see more from our Master of Microscopes, James check out Jam and Germs on Instagram.