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).
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.