Microbe Profiles: Rotifers
Animals live in the tundra, the forests, and in drops of water
The world of microbes does not only consist of single cells — although I’ll fight you if you claim my asexual, microscopic brothers are anything but fantastic. The unseen world also contains tiny animals, typically called microanimals or archaically ‘animalcules’. Rotifers are kind of special, because they are the smallest animals we know of on the face of the Earth.
They are named ‘rotifers’ (and nicknamed ‘wheel animalcules’) because of the rotation of cillia at the front of their bodies. As the history books tell, they were originally discovered by Antonie van Leeuwenhoek, the same guy who made the microscope, in the 1700s. I identify with rotifers on a personal level, as well as a professional one. With only about 1,000 cells per individual, they are relatively petite, slightly misshapen organisms who are just trying to make it in a paramecium eat paramecium world.
While they were first uncovered a few millennia ago, there’s no telling how long we’ve coexisted, or how much they predate our existence — although after studying their microbial cousins we can speculate that they evolved pretty early on. Rotifers have soft bodies, aside from a their hard jaws. This, and their incredibly small size, makes them hard to preserve in the form of fossils.
Phylogenetics and Reproduction (How Rotifers Fit Within the Tree of Life)
Rotifers are apart of the kingdom of animals, just like humans and dogs and cows — you get the idea. They, as such, they have male and female individuals within the population.
They reproduce sexually (and sometimes asexually — perhaps because males tend to be scarce in number), unlike most microbes, and while most genera lay eggs, some sources suggest species may give birth to live young.
Based on videos of this phenomenon, I must say it’s unclear whether the rotifer is ‘giving birth’ or simply reproducing asexually. Surprisingly, there is some pretty detailed information about the nuances of rotifers reproductive lives, including many of the the variations of sexual practices between species.
It was hard to find a credible source to verify this, but the consensus seems to be that rotifers have about 100 nuerons per individual. At such a small scale, it’s almost unimaginable that rotifers live lives so similar to our own!
Rotifers are a phylum of animals with shared characteristics. As we look a bit more specifically at their morphology and behavior, we can group them more exclusively into three classes: Monogononta, Bdelloidea, and Seisonidea.
Within these classes, there are numerous genera (singular: genus) and still more species within those genera.
Habitats
The habitat of rotifers is a special one. Found mainly in freshwater environments, they tend to live in thin films of water surrounding soil particles. They can also survive at the bottoms of lakes, in flowing streams and rivers, and — like most organisms — have made their mark on nearly all of the planet.
They can live in lichens on tree trunks, in mushrooms by fallen trees, in the rain gutters of human habitats, next to aquatic animals like crabs and lobsters, and even in waste treatment plants.
Most rotifers swim through their habitats, taking full advantage of their membrane crafted wheels, but others have a more peculiar method of travel. Rotifers at the bottom of lakes, for example, have a ‘looping’ method of travel and are described to alternate between their head and tail ends.
Others still remain nonmotile, anchored by tentacles to the bottom of these lakes.
Diets
Rotifers are often filter feeders— omnivores, one might argue. They feed off of their microscopic neighbors and any other thing that will fit into their mouths.
They have jaws, as we mentioned before, and mouths and, typically, fully formed digestive tracts. Some are parasitic, some live in colonies, but most live and feed in solitude.
They also can be cannibals, killing members of their own species to survive, but they typically stick to consuming phytoplankton, algae, and other organic molecules. They are then preyed upon by larger animals like shrimp.
Superpowers
The trend in microscopic superpowers is shocking, but also kind of expected. Microbes make the human population of 7 billion look insignificant, and this is mainly due to their brilliant ability to evolve and reproduce quickly, and adapt to their environments, preventing a change in environment from wiping them out completely. This also means that they don’t have particularly discerning tastes in real estate, and seem to live everywhere imaginable.
The superpower I’d like to highlight here is called cryptobiosis, a method of survival that slows metabolic activity, and allows Bdelloid rotifers to survive extreme drying or desiccation. Some rotifer eggs, especially those containing older embryos, can also survive desiccation, giving their species a few more survival points.
In this area, studies have found conflicting results, but there is some evidence that suggests rotifers experience slowed respiration when resources are scarce.
Out in the field
Rotifers are pretty easy to find and observe. You’ve got a pretty good chance of sweeping some up in moss on a tree, or soil in a freshwater stream. They have a habit of adhering to their environments (pipettes, slides, etc) so it’s best to move quickly. They’re pretty easy to disturb and will curl up into a little ball if poked, so try to avoid using slide covers. Some rotifers can be parasitic, but observing rotifers is fairly risk free and easy with a microscope.
My first time viewing the microscopic world in a single drop of water was, perhaps sadly, one of the happiest moments I can remember. I can only hope your observations are as enlightening as mine were.
Don’t take my word for it! View the full list of citations here.
If you want to learn more about the microscopic world, take a look at Microbe Profile on the tentacled Suctorians.
Hi! I’m flagellate. When I’m not swimming in agar, or writing about my favorite microbes, you can find me making up stories about the end of the world, and helping science get funded.
