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.
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