marine environments, mostly on the sea floor. About 500 species are found in fresh water and another 5,000 species are the terrestrial woodlice, which form the suborder Oniscidea. In the deep sea, members of the suborder Asellota predominate, to the near exclusion of all other isopods, having undergone a large adaptive radiation in that environment. The largest isopod is in the genus Bathynomus and some large species are fished commercially for human food in Mexico, Japan and Hawaii. Some isopod groups have evolved a parasitic lifestyle, particularly as external parasites of fish. They can damage or kill their hosts and can cause significant economic loss to commercial fisheries. In reef aquariums, parasitic isopods can become a pest, endangering the fish and possibly injuring the aquarium keeper. Some members of the family Cirolanidae suck the blood of fish, and others, in the family Aegidae, consume the blood, fins, tail and flesh and can kill the fish in the process. The World Marine, Freshwater and Terrestrial Isopod Crustaceans database subdivides the order into eleven suborders: Asellota – This suborder contains the superfamily Aselloidea, a group that contains most of the freshwater isopods in the northern hemisphere, and the superfamilies Stenetrioidea, Gnathostenetroidoidea and Janiroidea, which are mostly marine. The latter superfamily, Janiroidea, has a massive radiation of deepsea families, many which have taken bizarre forms. Calabozoida – A small suborder consisting of two marine species in the family Calabozoidae and one freshwater species in the family Brasileirinidae which is found in subterranean locations. Cymothoida – Chiefly marine isopods with over 2,700 species. Members are mostly carnivorous or parasitic. Includes the family Gnathiidae, the juveniles of which are parasitic on fishes. The previously recognised suborder Epicaridea is included as two superfamilies within this suborder and Cymothoida now includes part of the formerly recognised suborder Flabellifera. Also includes the former suborder Anthuridea, a group of worm-like isopods with very long bodies. Limnoriidea – Mainly tropical isopods, some of which are herbivorous. Microcerberidea – Tiny, worm-like isopods that live between particles on the bed of freshwater and shallow marine habitats. Oniscidea – Semi-terrestrial and terrestrial isopods fully adapted for life on land. There are over 4,000 species of woodlice inhabiting forests, mountains, deserts and the littoral zone. Phoratopidea – A single marine species, Phoratopus remex, which warrants its own suborder because of its unique characteristics. Phreatoicidea – Small suborder of freshwater isopods resembling amphipods, limited to