Temporal range: 300–0Ma
|Eurydice pulchra (Cirolanidae)|
Isopoda ("isopods") is an order of peracarid crustaceans, including familiar animals such as woodlice and pill bugs. The name Isopoda derives from the Greek roots ἴσος (iso-, meaning "same") and ποδός (podos, meaning "foot"). The fossil record of isopods dates back to the Carboniferous period (in the Pennsylvanian epoch), at least 300 million years ago.
Isopods are typically flattened dorso-ventrally, although many species deviate from this plan, particularly those from the deep sea or from ground water. Isopods lack an obvious carapace, which is reduced to a "cephalic shield" covering only the head. Gas exchange is carried out by specialised gill-like pleopods towards the rear of the animal's body. In terrestrial isopods, these are often adapted into structures which resemble lungs, and these "lungs" are readily visible on the underside of a woodlouse. Eyes, when present, are always sessile, never on stalks. They share with the Tanaidacea the fusion of the last abdominal body segment with the telson, forming a "pleotelson", and the first body segment of the thorax is fused to the head. The pereiopods are uniramous, but the pleopods are biramous.
Around 4,500 species of isopods are found in marine environments, mostly on the sea floor. Some 500 species are found in fresh water; and another 5,000 species are the woodlice in the suborder Oniscidea, which are thus by far the most successful group of terrestrial crustaceans. 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.
A number of isopod groups have evolved a parasitic lifestyle. The suborder Cymothoida is exclusively parasitic, while the polyphyletic suborder Flabellifera is partly parasitic. Cymothoa exigua, for example, is a parasite of the spotted rose snapper fish Lutjanus guttatus in the Gulf of California; it eats the tongue of the fish, and takes its place, in the only known instance of a parasite functionally replacing a host structure.
Diversity and classification 
Isopods belong to the larger group Peracarida, which are united by the presence of a special brood pouch for brooding eggs. There are around 10,215 described species of isopod, classified into eleven suborders.
Isopod larvae hatch as mancae, which resemble adults except for the lack of the last pair of pereiopods (thoracic legs). The lack of a swimming phase in the life cycle is a limiting factor in isopod dispersal, and may be responsible for the high levels of endemism in the order. As adults, isopods differ from other crustaceans in that they replace their exoskeleton (in the process called ecdysis) in two phases; this is known as "biphasic moulting".
- M. Schotte, C. B. Boyko, N. L. Bruce, J. Markham, G. C. B. Poore, S. Taiti & G. D. F. Wilson. "World List of Marine, Freshwater and Terrestrial Isopod Crustaceans". World Register of Marine Species. Retrieved October 7, 2010.
- Richard Brusca (August 6, 1997). "Isopoda". Tree of Life Web Project.
- Frederick R. Schram (1970). "Isopod from the Pennsylvanian of Illinois". Science 169 (3948): 854–855. doi:10.1126/science.169.3948.854. PMID 5432581.
- S. J. Keable, G. C. B. Poore & G. D. F. Wilson (October 2, 2002). "Australian Isopoda: Families". Australian Museum.
- R. C. Brusca & M. R. Gilligan (1983). "Tongue replacement in a marine fish (Lutjanus guttatus) by a parasitic isopod (Crustacea: Isopoda)". Copeia 3 (3): 813–816. doi:10.2307/1444352. JSTOR 1444352.
- Ronald L. Shimek (2002). "Pills, parasites, and predators; isopods in the reef aquarium". Reefkeeping 1 (4).
- Joel W. Martin & George E. Davis (2001). An Updated Classification of the Recent Crustacea (PDF). Natural History Museum of Los Angeles County. pp. 132 pp.
- Katie Drummond (April 1, 2010). "Cockroach of the Sea Wows the Web". AOL News.
- Media related to Isopoda at Wikimedia Commons
- Data related to Isopoda at Wikispecies