Oniscus asellus

Oniscus asellus was described as Oniscus Aſellus in the tenth edition of Systema Naturae by Carl Linnaeus in 1758. Numerous other names, many also in the genus Oniscus but two in the genus Porcellio, have since been identified as taxonomic synonyms of Linnaeus' species. The subspecies common in most of the world is O. asellus asellus, but in western France and southeastern Britain, a genetically distinct form of the common woodlouse has been identified and classified as the subspecies O. asellus occidentalis. Described in 1994 by David Bilton it tends to be smaller but more colourful, and has a slightly different body shape. The two subspecies are able to interbreed and produce hybrids which mix their physical characteristics. ==Distribution==

The common woodlouse is the most widespread species of woodlouse in the British Isles, both geographically and ecologically, being one of the "most pervasive" of Britain's wildlife. It is adapted equally well to northern areas as to southern ones, and has been recorded in every vice-county of England, Scotland, Wales, and Ireland. It is less common in the Mediterranean Basin, but is widespread in Northern and Western Europe, from Portugal to Sweden as far east as Ukraine, as well as in the Azores and Madeira; it has also been widely introduced in the Americas, ==Description==

General appearance The common woodlouse is one of the largest native woodlice in Britain, at up to long. It is relatively flat, and is a shiny brown-grey in colour, The juveniles have rough bumps and orange markings, leading them to sometimes be misidentified as Porcellio scaber. Moulting Pale patches are often visible on the back of Oniscus asellus; these are areas that store calcium, which is then used to reinforce the exoskeleton after a moult. In males, there are two testes, each comprising three "narrow lobes" with three corresponding into the vas deferens. Inside the lobes, germ cells develop; each lobe "may be divided into two principal regions of growth, composed of cells of different generations and of differing degrees of development." ==Ecology==

, Canada The common woodlouse occurs in a wide range of habitats, including some with little available calcium, but not dry ones. The species is found both inland and in coastal areas, chiefly in the countryside, but the species is also a successful synanthrope, and is frequent inhabitant of human spaces such as greenhouses, gardens, and parks. The common woodlouse is found in waste grounds, open woodlands, forests, and gardens, as well as in grasslands, scrub, and around buildings. The plurality (22%) of records examined in one study were found on road verges, although it has also been collected from cliff faces and quarries. It most commonly dwells in moist environments under rocks or dead wood, but has also been recorded from litter, human constructions, and garbage. It is especially prevalent in rotting wood, a tendency which allows it to inhabit areas otherwise unfriendly to woodlice such as moorlands. According to Stefan Gates, they have a sweet meat with taste similar to crab or shrimp, and considerable umami flavour. Heavy metal accumulation . Isopods are known to store high levels of lead, copper, zinc, and cadmium from contaminated environments in their hepatopancreas, an organ which is believed to be "the most important [...] to monitor the effects of heavy metal pollution" in some isopod species, including O. asellus. In fact, a 1982 paper noted that the concentrations of these four elements in the hepatpancreata of O. asellus living in contaminated environments "are among the highest so far recorded in the soft tissues of any animal". It is thought that retaining non-essential elements such as cadmium takes less energy than preparing to excrete them. It is also believed that woodlice that live in contaminated environments have larger hepatopancreata, allowing them to process more material, based on a positive correlation between the level of contamination of the leaf litter and the dry weight of the hepatopancreata of woodlice living there. A 1990 study comparing the abilities of O. asellus and Porcellio scaber showed that O. asellus dumped zinc rapidly whereas P. scaber retained it, while O. asellus retained much more cadmium and lead – on average about five times as much. Copper was accumulated by both species. Conservation The subspecies O. asellus occidentalis is threatened by population fragmentation and by hybridisation with O. asellus asellus. In Britain, it is classified as Near Threatened. ==See also==