Rotifers - Some of the smallest animals on earth

Rotifers are some of the smallest animals on the planet. Most measure under 500 microns, but their size can range from only 50 microns to around 2 mm (2000 microns). They are some of the most common microscopic animals, living all over the world in all kind of environments, like freshwater, saltwater, in moss on trees and in the soil underneath your feet. Rotifers are made up of roughly 1000 cells, some of which are specialized to form organ systems. Despite their incredibly small size they have eyes, a small brain and nervous system, and a dedicated digestive system just like us.

Rotifers are more commonly referred to as wheel animals due to the presence of structures that resemble turning wheels on their heads. These structures are called the corona, from the Latin word for a crown, and are made up of small hair-like projections called cilia. These ciliated structures are actually not turning at all, but beating in a synchronized movement to create a current used for both movement and feeding. They feed on small organic particles, algae, bacteria and other organisms smaller than 10 microns in size (roughly the size of our blood cells).

Some rotifers, like species from the class Bdelloidea, are able to enter an inactive and protective state just like tardigrades called cryptobiosis. This state lets the animals survive both drought and other unfavorable conditions. When they get rehydrated, they “come back to life” and continue on like nothing had happened.