Shell shape variation in Littorina saxatilis and L.arcana: a case of character displacement?

Variation in shape of the shell of Littorina saxatilis and Littorina arcana has been investigated using shells from four shores where the species co-occur and seven where the former only is present. Samples were taken from shores along the south-west and south coasts of Britain, between Cardigan Bay and Kent. Measurements of eight shell variables and of the operculum area were analysed using canonical variate analysis. The results of the analysis suggest that the shells can be separated into three groups, namely L. arcana, L. saxatilis where it is sympatric with L, arcana, and L. saxatilis where it is allopatric from L. arcana. The separation of the two groups of L. saxatilis is based primarily on their relative globosity, whereas the separation of L. saxatilis from L. arcana is based primarily on the relative size of the operculum and width of shell whorl one (the first above the body whorl). Sexual dimorphism has been considered and shown not to contribute significantly to the between-shore pattern of variation observed. The relationships between important variables were further investigated using ANCOVA, and the relationship of foot area to the shell variables was also analysed at four of the shores. It is concluded that there is evidence of character displacement in L. saxatilis, shown by an increase in globosity in the presence of L. arcana, and that this may partly explain the continuing uncertainty over the taxonomic and ecological identity of members of the rough periwinkle group.