Rotifers are microscopic animals in the phylum Rotifera, which encompasses over two thousand species. These tiny animals are quite unique because they demonstrate incredible complexity in comparison with organisms of a similar size. They can be found in aquatic environments all over the world, living a wide range of lifestyles. If you have a microscope, you can probably see some rotifers for yourself by taking a water sample from a neighboring stream or pond; you will probably also find some other microscopic organisms floating around.
The distinguishing feature of a rotifer is its corona, a collection of cilia shaped like a wheel around the head end of the organism. As the cilia move, they create a current which sucks in food for the rotifer and helps it to navigate. Depending on the type of rotifer, the organism may have one or more trailing tails or feet. Early observers of rotifers in the 1700s referred to the animals as “wheel animalcules,” in a reference to the corona.
Many rotifers are transparent, allowing people to see their internal structures. The organisms have simple brains and full digestive tracts, surrounded by a shell which can vary in shape, size, and hardness. These very small animals can live alone or in groups, as freewheeling organisms or as parasitic ones, and in a variety of fresh and saltwater environments. This incredible diversity has probably allowed rotifers to survive for millions of years, although they are hard to find in the fossil record due to their small size and soft body structures.
Some common types of rotifers include sessile, planktonic, bdelloid, and loricate rotifers. Sessile rotifers attach themselves to various aquatic plants, while planktonic rotifers are free floating, like the plankton they are likened to. Loricate rotifers have hard to firm shells, while bdelloid rotifers have soft bodies and they can be found on ponds and in moist environments like beds of moss and swamps. As one might imagine, some rotifers are capable of surviving in very intense conditions which would kill other organisms; some are able to stay in a state of suspension in extreme cold or dryness, for example.
These diverse creatures reproduce in a range of ways. Some lay eggs, while others reproduce through parthogenesis, meaning that they produce eggs asexually. The offspring are usually exact clones of their parents, although some rotifers are capable of asexually producing simple male offspring which can mate with females, resulting in fertilized eggs which drift through the water until they hatch, contributing to the spread of the species.