A major taxon of the Crustacea containing small bivalved animals 0.004–1.4 in. (0.1–33 mm) long, with most between 0.04 and 0.08 in. (1 and 2 mm). They inhabit aquatic environments in nearly all parts of the world. Semiterrestrial species have been described from moss and leaf-litter habitats in Africa, Madagascar, Australia, and New Zealand, and from vegetable debris of marine origin in the Kuril Archipelago. In the oceans, ostracodes live from nearshore to abyssal depths, some swimming and others crawling on the bottom; several are adapted to estuaries of rapidly changing salinity. In fresh water, ostracodes inhabit lakes, ponds, streams, swamps, caves, and ephemeral little standing bodies of water. Of the more than 2000 species extant, none is truly parasitic and most are free-living. However, a few fresh-water and marine forms live commensally on other animals: among the Podocopina, Entocythere clings to the gills of crayfish and Sphaeromicola and Paradoxostoma rostratum to appendages of isopods and amphipods; among the Myodocopina, Vargula parasitica and Sheina orri are found on the gills of sharks and rays. Ostracodes themselves are parasitized; some marine species are infested with parasitic isopods and copepods, and some fresh-water species serve as intermediate hosts for a cestode (the adult of which parasitizes the black duck) and for an acanthocephalan (the adult of which parasitizes the black bass). Most ostracodes are scavengers, some are herbivorous, and a few are predacious carnivores. Exceptional biological features are known; there are myodocopine ostracodes that produce bioluminescence, and some species of podocopines form a secretion from spinning glands to enable them to climb polished surfaces.