Intraspecific variation in the terrestrial isopod Oniscus asellus Linnaeus, 1758 (Crustacea: Isopoda: Oniscidea)

A study of intraspecific variation in one of the most widespread and abundant terrestrial isopods in Europe, Oniscus asellus , is presented. It is concluded that previous attempts to describe intraspecific taxa in this species are invalid, but that the species is clearly divisible into two subspecies which differ in a number of features, including male sexual modifications. The nominal subspecies, Oniscus asellus asellus Linnaeus, 1758, is described in detail, and a neotype, collected from Uppsala, Sweden, where Linnaeus lived and worked, is designated. Oniscus asellus occidentalis ssp. nov . is also described, and reported as occurring in the far west of the species' range, in the British Isles, Ireland and France. Characters useful in distinguishing these two taxa are indicated and figured. Differences in the ecology of the two taxa are also noted, occidentalis appearing to be a relict Atlantic taxon, largely restricted to damp woodland. The existence of populations intermediate between asellus and occidentalis is noted, and the features of these populations discussed in detail. A combination of geographical, environmental and laboratory breeding data suggest that these intermediates arise through hybridization between the two subspecies, this process of hybridization being accelerated by human activity, which brings the two taxa into frequent contact. It is suggested that Oniscus asellus may provide an excellent model system for the study of the dynamics of hybrid zones.