Rotifers

Rotifera is one of the largest micro-invertebrate phyla, in terms of both biomass and number of species. Its members are major components of freshwater and coastal marine ecosystems throughout the world, and are the chief non-arthropod component of most freshwater pelagic communities. Most rotifers are smaller than 1mm, but have ganglia, muscles, photo-, chemo-, and tactile sensory organs; structures for crawling, feeding, and swimming; digestive and secretory organs; and ovaries. The two major rotifer groups, Monogononta and Bdelloidea, are each significant for their unusual reproductive modes: bdelloids appear to be obligately asexual, reproducing only through mitotic division of germ cells; monogonont rotifers are facultatively sexual, generally reproducing through mitotic division of germ cells but entering a meiotic sexual phase in response to environmental cues.
The phylum is allied with other micro-invertebrate groups such as Acanthocephala, Gnathostomulida, Micrognathozoa, and Cycliophora in the superphylum Gnathifera, a sister- or basal group to the Lophotrochozoa. Unlike other gnathiferan fauna, many rotifer species can easily be cultured in axenic conditions in quantities suitable for large-scale biochemical and molecular genetic studies, making rotifers the optimal model system for studying this basal animal assemblage. Among rotifers, the most well-studied are the euryhaline rotifers of the Brachionus plicatilis species complex. Their widespread distribution and ease of culturing make this group a useful model system for studies of population dynamics, speciation, the evolution of sexual reproduction, and ecotoxicology. The complex is also an important live food for the initial stage of larval rearing of marine fishes.