Rotifers are microscopic aquatic animals of the phylum Rotifera. Rotifers can be found in many freshwater environments and in moist soil, where they inhabit the thin films of water that are formed around soil particles. The habitat of rotifers may include still water environments, such as lake bottoms, as well as flowing water environments, such as rivers or streams.
Rotifers are also commonly found on mosses and lichens growing on tree trunks and rocks, in rain gutters and puddles, in soil or leaf litter, on mushrooms growing near dead trees, in tanks of sewage treatment plants, and even on freshwater crustaceans and aquatic insect larvae.
Rotifers are multicellular animals with body cavities that are partially lined by mesoderm. These organisms have specialized organ systems and a complete digestive tract that includes both a mouth and anus. Since these characteristics are all uniquely animal characteristics, rotifers are recognized as animals, even though they are microscopic. Most species of rotifers are about 200 to 500 micrometers long.
The name ‘rotifer’ is derived from the Latin word meaning ‘wheel-bearer’; this makes reference to the crown of cilia around the mouth of the rotifer. The rapid movement of the cilia in some species makes them appear to whirl like a wheel.
Rotifers are primarily omnivorous, but some species have been known to be cannibalistic. The diet of rotifers most commonly consists of dead or decomposing organic materials, as well as unicellular algae and other phytoplankton that are primary producers in aquatic communities. Such feeding habits make some rotifers primary consumers. Rotifers are in turn prey to carnivorous secondary consumers, including shrimp and crabs.