GB rarity status: Nationally Scarce
A morphologically distinct form of Oniscus occurs has been discovered in south-west England, which is smaller, more brightly coloured and with a more highly arched body. Molecular studies have shown this to be a genetically distinct taxa of ancient divergence (Bilton, et al, 1999).
Since the two forms can interbreed, to produce morphologically intermediate hybrids, the south-western taxon was described as a sub-species O. asellus ssp. occidentalis (Bilton, 1994) rather than a full species. It is only reliably identified from a male specimen, where the tip to the 1st endopod is forked (simple in O. a. asellus).
Distribution and Habitat
Pure populations of O. a. occidentalis occur in south-western England, south Wales and the south coast of Ireland. Beyond this intermediate forms of hybrid origin occur sporadically throughout England (and probably Ireland).
O. a. occidentalis favours rural semi-natural damp woodland, wetland and rank grassland (whereas O. a. asellus readily colonises drier habitats and synanthropic sites).
This summary is based on the detailed account in Gregory (2009).
Gregory, S. (2009) Woodlice and Waterlice (Isopoda: Oniscidea & Asellota) in Britain and Ireland. Field Studies Council/Centre for Ecology & Hydrology.
Bilton, D.T., Goode, D. & Mallet, J. (1999). Genetic differentiation and natural hybridization between two morphological forms of the common woodlouse, Oniscus asellus Linnaeus, 1758. Heredity, 82: 462-469.
Bilton, D.T. (1994). Intraspecific variation in the terrestrial isopod Oniscus asellus L. Zoological Journal of the Linnean Society, 110: 325-354.