Inferring fast ecotypic divergence in a protected marine area: comparing QST and FST patterns in Littorina saxatilis subpopulations from Cíes Islands in Spain

The intertidal snail Littorina saxatilis is well known for its remarkable variety of locally adapted populations. A unique L. saxatilis habitat is represented by the sheltered rocky environment found in the natural coastal lagoon of the Cies Islands (NW Spain). The present-day characteristics of this habitat are relatively recent (~ 70–150 years), some of these have been human induced. In this study, we indirectly tested local adaptation resulting in ecotypic variation in this habitat by estimating the additive genetic differentiation between populations (QST) for morphological shell traits in families of embryos. We compared the Qst and Fst (AFLP loci) between populations of the previously described Crab ecotype (outside the lagoon) and the new Cies lagoon ecotype (inside) as a proxy for adaptation. For several traits (RW1 and RW2; associated with a change in size of shell aperture) the QST was higher than corresponding FST, which indeed suggest that natural selection could be the force behind the apparent phenotypic divergence observed between the Crab and the Cies lagoon ecotype. The putative process of local adaptation occurring in the Cies lagoon ecotype (favoring a reduction of shell aperture) might be considered a case of fast phenotypic evolution driven by anthropogenic effects and with important conservation implications. Further genetic and ecological studies must be carried out to advance our knowledge in such an interesting system.