Habitat-dependent ecotype micro-distribution at the mid-shore in natural populations of Littorina saxatilis

On the NW coast of Spain, there is a hybrid zone in natural populations of L. saxatilis, where the largest ecotype is adapted to the desiccation and beat stress of the upper shore, whereas the smaller one is adapted to the wave impacts of the lower shore. The two ecotypes meet and hybridise in a mid-shore area producing intermediate individuals or hybrids. It has been postulated that this hybrid zone is maintained by a selection-gradient model that assumes habitat-dependent selection for the different environments of upper and lower shore areas. In this study, we focus on the hybrid (mid-shore) area, where a transition occurs between upper-shore and lower-shore environments, in order to ascertain whether the habitat-dependent selection is maintained at the micro-habitat scale. We present data on snail density at several levels of the mid-shore (varying accordingly in the relative frequency of mussels and barnacles) obtained for three seasons in three consecutive years and three localities. In the mid-shore, the abundance of the RB ecotype increases with increasing tidal height, whereas the abundance of the SU ecotype increases with decreasing tidal height, suggesting that habitat-dependent selection is maintaining the ecotype micro-distribution.