Assortative mating between two distinct micro-allopatric populations of Littorina saxatilis (Olivi) on the northeast coast of England

Size assortative mating is a common invertebrate mating pattern and is usua lly accompanied by male and female sexual selection, and these three behavi ours can contribute to reproductive isolation. Two distinct populations of the marine prosobranch Littorina saxatilis, H and M, occur within 15 m of e ach other on the same shore. Previous studies have demonstrated that these two forms have different reproductive strategies and that the rare hybrids between the two forms Show evidence of reproductive dysfunction and hence a re less fit than the assumed parental forms. In both populations, female sh ell height war shown to be a predictor of the number of embryos contained w ithin the brood pouch. The mean shell height of the M population was signif icantly larger than that of the H population, and the M population matures at a larger shell size than the H population. The two populations show complete assortative mating to type in the field, and occupy different microhabitats on the Same shore. Therefore, laboratory -based experiments were performed to determine if assortative mating was ma intained in sympatry and also to determine the effect of population density on mate choice. The males of both populations showed sexual selection for female size, choosing to mate with females approximately 10% larger than th emselves from an assortment of female sizes. The M population showed comple te assortative mating to type, irrespective of the density of H and M femal es, whereas at low densities the H males did occasionally mate with M femal es. The role of assortative mating and reinforcement (due to natural select ion acting against the less fit hybrids), in maintaining the partial reprod uctive barrier between the two populations is discussed.