Individual variation in the seasonal reproduction of the terrestrial isopod Armadillidium vulgare Latr. (Crustacea, Oniscidea)

Under particular conditions of photoperiod and temperature, Armadillidium vulgare females, originating from a single population, might exhibit individual differences in the onset of reproduction and duration of the breeding period. In a population issued from a strain from middle latitudes, some females underwent only one parturial moult (northern tendency) and others three parturial moults (southern tendency). Females with an atypical northern phenology are the most numerous and tend to be found near the Danish population. In the latter, there is an asymmetrical response to laboratory selection (favourable to females with a longer breeding period). The asymmetrical variation in atypical individuals acts as a safety device against the unpredictability of the environment. The adaptation of this species, originally from the Mediterranean periphery, to a northern environment has led to a reduction in its capacity to breed over long periods of time. Populations from middle latitudes can undergo one or several parturial moults which enables the species to successfully colonize even far-away countries. These intrapopulation differences have an essential role and explain why Armadillidium vulgare is one of the most widely distributed species among Oniscidea.