Population demographics of native and newly invasive populations of the green crab Carcinus maenas

Green crabs Carcinus maenas (L.) are native to north-western Europe, but have been spread globally by humans during the last 200 yr. Reproductively viable populations have been present for < 10 yr in British Columbia, Canada. In the present study, C. maenas were collected from 2 geographically separated locations, Anglesey (UK) and British Columbia (Canada), to compare body-size and colour distributions between native and newly invasive populations. Crabs were captured using baited traps and collected by hand at both intertidal and shallow subtidal elevations. Crabs from British Columbia were significantly larger than those from Europe. The largest male, 101.1 mm, and the largest female, of 85.4 mm carapace width, were both captured in British Columbia. The native populations showed a higher frequency of red-coloured crabs than the introduced population, which consisted predominately of green-coloured male crabs. Green-coloured integuments are typical of individuals in the early stages of intermoult. Accordingly, the high frequency of large, green-coloured C. maenas in British Columbia suggests that individuals in this population have an atypically high growth rate and achieve a larger body size and, hence, potentially greater fecundity. Moreover, the scarcity of small C. maenas in British Columbia may indicate that the existing population comprises only the first or second generation of recruits. The observed differences in body size and colour distribution are perhaps indicative of release from an as yet undetermined growth-limiting factor (possibly parasites) and provide a unique opportunity to study the dynamics of a newly invasive population as it recruits and matures.