Sometimes it seems like evolution has a sense of humour. Can you imagine a creature with wheels on its face, or anywhere for that matter?
The Rotifer doesn’t actually have wheels on its face. There is a reason animals don’t have wheels, in case you were wondering. Basically, it has to do with the way evolution works and the way wheels work. Evolution happens through trial and error (a wheel must be perfect in order to function), and a wheel cannot be attached to the axis it’s rotating on, so the body wouldn’t be able to supply it with nutrients. The rotifer’s spinning effect is actually created by tiny hairs called cilia that move around rapidly, creating a vortex in the water where the rotifer lives. Think of one of those signs with rows of lights where it looks like the lights are moving or chasing each other around the edge, but they are actually just flashing in a sequence. Here’s a video I took with my iPhone at the microscope last year to show a rotifer’s mouth in action.
Much more is known about aquatic rotifers than terrestrial (soil) ones, but I am more familiar with the soil ones since my work is in soil biology. Having said that, the soil rotifers (and other microorganisms in soil) are not much different than those that live in water. They are still aquatic creatures, since they make their home in the super thin layer of water surrounding moist soil particles. The soil doesn’t have to be saturated for these animals to function, but when it does get too dry they will simply go dormant until things improve. Rotifers are actually studied quite a lot and have been noted for their unique ability to survive radiation. Here is a fascinating article about rotifer survival.
What role do rotifers play in your garden?
Rotifers are filter feeders, preying on bacteria, protozoa, and detritus, aka decaying organic material. That means they help recycle nutrients in the soil and it’s good to have them in your garden.
And here is my drawing of a rotifer; the third piece in my soil life illustration series:
The cilia (hairs) on the rotifer’s face move so fast in reality that I couldn’t produce or find any good imagery showing how they actually work in detail. It’s kind of like when you try to take a video of a propeller or a fan and it looks like the blades just vibrate in place or are slowly moving backwards. The videos make it seem like the rotifer literally has a wheel with small hooks that kind of looks like a knitting loom, but I’m not sure that’s how it really is.
I’m also not clear on how exactly the cilia are arranged so there was a bit of guesswork involved. There are over two thousand different species of rotifers to complicate things even further. In some images it looks like a dense mop, in others it looks like sets of tiny rows that run perpendicularly around the ring, and in other cases it looks like single cilia in a simple row. I chose to draw a generalized bdelloid rotifer and used the simplest cilia concept that looks the most like how I’m used to seeing them. I’m still not convinced that this visualization is exactly correct, though.
In the microscope I usually notice a distinct movement of debris caused by the rotifer’s spinning before I find the animal itself, so I added some bits flowing around the mouthparts to try to demonstrate that a bit. I also included an amoeba (looks like a piece of translucent gum) to the right of the rotifer, and a small flagellate in the top left corner. The soil itself is the most tedious part of these drawings so I like to try and add some little details here and there to break the monotony.
Now I just need to decide what the next subject will be for this series. I’m thinking either a nematode trapped by a fungus or an amoeba swallowing something up.