Genetic variation for shell traits in a direct-developing marine snail involved in a putative sympatric ecological speciation process

Populations of the marine gastropod Littorina saxatilis from exposed rocky shores of NW Spain provide one of the few putative cases of sympatric ecological speciation. Two ecotypes with large differences in shell morphology and strong assortative mating are living at different vertical levels of the shore separated by a few meters. It has been hypothesized that shell size is the main determinant for the reproductive isolation observed between the ecotypes, and that several shell shape traits are subject to divergent natural selection and are responsible for the adaptation of each ecotype to its respective habitat. Using embryos extracted from wild females we obtain estimates of genetic variation for shell size and shape and compare them with those from neutral molecular markers. Estimates of heritability are significantly larger for the ecotype found in the upper shore than for that in the lower shore, in concordance with a similar result observed for heterozygosity of neutral markers. The large genetic differentiation between ecotypes for the shell traits, contrasting the smaller close to neutral differentiation between populations of the same ecotype, supports the implication of the traits in adaptation.