Isopoda

Isopoda Temporal range: 300–0 Ma PreЄ Є O S D C P T J K Pg N
Eurydice pulchra (Cirolanidae)
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
Subclass:	Eumalacostraca
Superorder:	Peracarida
Order:	Isopoda Latreille, 1817
Suborders
Asellota Calabozoida Cymothoida Limnoriidea Microcerberidea Oniscidea Phoratopidea Phreatoicidea Sphaeromatidea Tainisopidea Valvifera

Isopods are 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").^[1] The fossil record of isopods dates back to the Carboniferous period (in the US Pennsylvanian epoch), at least 300 million years ago.^[2][3]

Description

The woodlouse Oniscus asellus from the side

Isopods are relatively small crustaceans with seven pairs of legs of similar size and form, ranging in size from 300 micrometres (0.012 in) to nearly 50 centimetres (20 in) in the case of Bathynomus giganteus.^[1] They are typically flattened dorso-ventrally, although many species deviate from this plan, particularly those from the deep sea or from ground water.^[1] Isopods lack an obvious carapace, which is reduced to a "cephalic shield" covering only the head.^[4] 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.^[1] Eyes, when present, are always sessile, never on stalks.^[4] They share with the Tanaidacea the fusion of the last abdominal body segment with the telson, forming a "pleotelson",^[4] and the first body segment of the thorax is fused to the head. The pereiopods are uniramous, but the pleopods are biramous.^[4]

Ecology

Anilocra (Cymothoidae) parasitising Spicara maena, Italy

Around 4,500 species of isopods are found in marine environments, mostly on the sea floor.^[2] 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.^[2] 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.^[2]

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.^[5]

In marine and reef aquariums, parasitic isopods can become a pest, endangering both the fish and the aquarium keepers.^[6]

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,^[1] classified into eleven suborders.^[7]

Development

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.^[2] 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".^[1]

References

External links

	Crustaceans portal
	Arthropods portal

• Katie Drummond (April 1, 2010). "Cockroach of the Sea Wows the Web". AOL News. http://www.aolnews.com/weird-news/article/cockroach-of-the-sea-wows-the-web/19423132. 
• Media related to Isopoda at Wikimedia Commons
• Data related to Isopoda at Wikispecies