Mate Choice in Molluscs of the Genus Littorina (Gastropoda: Littorinidae) from White Sea

Abstract

We studied the reproductive behaviour associated with mate choice by size in three species of White Sea molluscs of the genus Littorina—L. saxatilis, L. obtusata and L. fabalis. Three behavioural patterns were revealed after a comparative analysis of the behaviour of the male snails. The males of L. obtusata, regardless of their size, could mate with all females but preferred to mate with larger ones. The males of L. fabalis chose females of their own size or larger. The males of L. saxatilis mated with females of approximately their own size. In the first case (L. obtusata), there is a free exchange of genes between all the size groups in the population. In the second case (L. fabalis), the gene transfer from small males to females of any size is free but the gene transfer from large males to small females is limited. In the third case (L. saxatilis), the gene flow is limited between groups of individuals of dramatically different size. Thus, in the case of L. saxatilis and L. fabalis, to varying degrees restriction of random mating associated with size assortativity in choosing a mating partner was expressed, which under certain circumstances (divergence of individuals in different econiches, the presence of ecotypes differing in size, lack of stronger natural selection in the other direction) can favour sympatric speciation. The males of all the three periwinkles’ species preferred larger females but sexual selection in favour of larger females was found only in L. obtusata. In the other two species, there were no statistically significant size differences between copulating and non-copulating females. This appears to be due to the occurrence of size-assortative mating in these species, which limits the ability of males to choose larger female partners.