The 1,500 to 2,000 species in the phylum Rotifera, like other members of the kingdom Animalia, are multicellular, heterotrophic (dependent on other organisms for nutrients), and lack cell walls. But rotifers possess a unique combination of traits that distinguish them from other animals, including bilateral symmetry and a pseudocoelom , a fluid-filled body cavity between two different layers of embryonic tissue. The pseudocoelom serves as a sort of circulatory system and provides space for a complete digestive tract and organs. It is also found in the closely related phylum Nematoda, a very common group of roundworm species. Unlike nematodes, which tend to live in moist soil, rotifers generally inhabit fresh water, although some species can be found in salt water or wet soil. Rotifers (Latin for "wheel bearers") have an unusual mode of transportation: a crown of beating cilia surrounds their mouth, propelling the animal head-first through the water. Rotifers eat protists, bits of vegetation, and microscopic animals (such as young larvae), which they suck into their mouths with the vortex generated by their cilia. Their jaws are hard and their pharynx is muscular, allowing them to grind up their food.
Another unusual rotifer trait is parthenogenesis, an asexual mode of reproduction in which females produce unfertilized eggs which are essentially clones of the mother. Some parthenogenetic species are completely female, while others produce degenerate, or poorly developed, males whose sole purpose is to fertilize eggs under stressful environmental conditions. Fertilized eggs are more resistant to drying out than unfertilized eggs. When conditions are favorable again, females return to producing unfertilized eggs. Because parthenogenesis is rare, rotifers provide valuable clues to biologists trying to understand the evolution of sexual reproduction, which remains one of the great unsolved mysteries of evolution.
Most rotifers are between 0.1 to 0.5 millimeters (0.004 to 0.02 inches) long, and generally contain only a few hundred cells. Another unusual characteristic of the phylum is that the number of cell divisions during development is fixed. This means that the animal is unable to grow new cells in response to damage. One might view this inflexibility as a drawback, but the fact that most of the rotifer's closest relatives do not share this trait suggests that the rotifer's recent ancestors were capable of regeneration . This means that rotifers could have inherited the ability to regenerate, but did not. Perhaps the ability to regenerate has costs as well as benefits, and the costs outweighed the benefits for the ancestor of the rotifers. Such costs are that regeneration requires energy, cell division, and can result in cancer-causing mutations.
Both asexual reproduction and deterministic cell division are examples of traits that might be considered "primitive" because they are simpler or tend to appear in older taxa. Yet, by examining a phylum like Rotifera and understanding its evolutionary history, we realize that there are situations in which apparently "primitive" traits replace "advanced" ones. A complex trait is not necessarily better than a simpler one. Each has its place in the diversity of life.
see also Phylogenetic Relationships of Major Groups.
Campbell, Neil A. Biology, 2nd ed. Redwood City, CA: The Benjamin/Cummings Publishing Company, Inc., 1990.
Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishers, 1989.
Rotifera, phylum of predominantly free-living, microscopic, aquatic or semiterrestrial pseudocoelomates. Each rotifer has a head bearing a crown of cilia, the corona, at the anterior end; most rotifers feed with the aid of currents generated by the coronal cilia. A posterior foot, often equipped with two or three toes, contains adhesive glands permitting temporary attachment to objects. Unique grinding jaws are found in the pharynx, and an esophagus, stomach, and intestine can be distinguished. The excretory system consists of ciliated cells, called flame cells, that move collected liquids into two coiled tubes called protonephridia; these tubes open into a contractile bladder. The reproductive system is simple, consisting in the female of ovary, yolk gland, and oviduct, and in the male of testis and sperm duct. The intestine, bladder, and reproductive ducts unite to form a cloaca.
Rotifers, of which there are about 1,500 known species, are widely distributed in freshwater and marine habitats; they also live in the soil, in mosses, and associated with lichens on rocks and trees. A few are parasitic. Most feed on bacteria, algal cells, small protozoans, or organic detritus. As a rule, only female rotifers are seen; in some species the males have never been observed. Diploid eggs develop parthenogenetically, i.e., without fertilization, to produce females. Under some conditions, haploid eggs are produced; these develop parthenogenetically into males or can be fertilized, developing into dormant female embryos with heavy shells (resting eggs). Many species can survive in a dry form for long periods of time, emerging from a dormant state and becoming active when moisture is available.