Rotifers are pretty animals, transparent with one or two red eye spots and dancing about on two toes like ballerinas, with a crown of cilia that twirl as they dance. Don't be fooled though, as they are actually fierce little predators, much feared in their microscopic communities. Most are female. In some species, males have never been found and the females reproduce parthenogenetically. As Meglitsch (1972) delightfully put it: "The study of rotifers is a study of ladies, sometimes beautiful, often capricious and always fascinating." The name Rotifer comes from the latin and means 'wheel-bearer' in reference to their ciliated crowns.
Rotifers are tiny, usually only a few hundred microns in length, but not single celled. They are classed as pseudocoelomates because they have a body cavity that separates the digestive tract from the muscles and which develops when the mesoderm next to the gut disappears during development. They are unsegmented and have a complete digestive system from the mouth to a cloaca, which acts to void wastes and deposit eggs. The pharynx contains jaws for holding and eating prey caught by the ciliated 'corona' above the mouth. Much of rotiferan taxonomy is based on the different types of pharynxes found in this phylum.
Rotifers are found in most freshwater ecosystems. They can also be found in moist soils, mosses and lichens and a few are found in salt water. An examination of pond water under a microscope will generally reveal these animals. They are related to roundworms and nematodes but are the least 'worm-like' in form of the pseudocoelomates. The crown and the two toes are their most identifiable features. Their bodies are transparent so it is possible to watch them catch their prey, usually protozoans, algae and detritus, chew it, swallow it and digest it. One can also see the eggs in their fat little bodies. Some species are almost flowerlike and hardly ever move, having secreted a glue to hold them in place, while others move or swim about or crawl in the mud. My favorites are the dancers on those two little toes.
Rotifers are also prey, being consumed by small freshwater crustaceans and fish. They are nutritious and easy to grow so are now cultivated as a food item in some aquaculture businesses. One advantage of rotifers is their ability to survive drying out in a state known as cryptobiosis. The eggs are also resistant to desiccation and can survive in the soil if their pond dries out and re-emerge when the rains return.
Rotifer sex life is unusual. Although they can reproduce sexually, in many cases they never do. The females of some species always produce unfertilized eggs that develop into daughters. In other species, the unfertilized eggs may develop into either females or small males who cannot feed themselves and whose only purpose is to fertilize females to produce true zygotes and add some diversity to the population. These little males die soon after reproduction. Production of males often occurs when conditions are becoming unfavorable and the fertilized zygotes seem to be particularly hardy and designed to survive the winter or the dry season and hatch when conditions improve.
About 2000 species of rotifers are known worldwide. They are divided into three classes and appear to be most closely related to the Phylum Acanthocephala or spiny headed worms. If you are unfamiliar with these lovely little beasts check out the photos on this website: http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/wimsmall/rotidr.html
References: Buchsbaum, R. 1968. Animals Without Backbones An Introduction to the Invertebrates. Penguin Books.
Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.