Oniscus asellus, the common woodlouse, or common shiny woodlouse^[2] is one of the largest and most common species of woodlouse native to the British Isles and Western and Northern Europe, growing to lengths of 16 mm and widths of 6 mm.

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.^[3] It is rare in the Mediterranean Basin, but is widespread in Northern and Western Europe, as far east as Ukraine, as well as in the Azores and Madeira; it has also been widely introduced in the Americas,^[1] predominantly in Mexico and in the United States, east of the Mississippi River and west of the Rocky Mountains.^[2]

The common woodlouse occurs in a wide range of habitats, including some with little available calcium, but not dry ones. It is The species is found both inland and in coastal areas, chiefly in the countryside, but the species is also a successful syanthrope, 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.^[3]

The common woodlouse is one of the largest native woodlice in Britain, at up to 16 mm (0.63 in) long.^[4] It is relatively flat, and is a shiny brown-grey in colour,^[3] but paler-coloured specimens sometimes occur.^[5] The juveniles have rough bumps and orange markings, leading them to sometimes be misidentified as Porcellio scaber.^[5]

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.^[4] Moulting occurs in two halves, with the rear half moulting before the front half. The exuvia (the thing that is being moulted) is often consumed by the animal after moulting.^[4]

The subspecies common in most of the world is O. asellus asellus, but in western France^[6] and southeastern Britain, a genetically distinct form of the common woodlouse has been identified and classified as a subspecies. O. asellus occidentalis was described in 1994, 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.^[7]

The subspecies O. asellus occidentalis faces multiple threats: its populations are fragmented, and it is also threatened by hybridisation with O. asellus asellus. In Britain, it is classified as "Near Threatened".^[8]

• Crustacean
• List of woodlice of the British Isles

• Media related to Oniscus asellus at Wikimedia Commons