Assortative mating between phenotypes of the intertidal snailLittorina brevicula: A putative case of incipient speciation?

A population of Littorina brevicula from Amakusa (Japan) which has been morphologically described as monomorphic, splits into an upper and a mid shore sub-population during the mating season. It has been suggested that this behavioural dimorphism in migration has a genetic basis. In the present study we sampled mating and non-mating individuals from both upper and mid shore populations to study mating behavioural components for 9 different shell measurements. Multivariate analyses of morphological data revealed two morphotypes (slender, S, and globose, G, shells) mostly distinguished by shell length (SL) and (W2/W1), and present in both upper and mid shore samples. The two morphotypes mated assortatively in the upper shore, but not in the mid shore. A few alternative mechanisms are plausible, but natural selection working on genetic variability seems the best explanation at present. Moreover, these phenotypic polymorphisms are partially coupled, i.e. morphotype and subpopulation affiliation (migratory ability) was positively correlated (r=0.363). The importance of this polymorphism is discussed in relationship to speciation.