Extensive micro-geographical shell polymorphism in a planktotrophic marine intertidal snail

Abstract

Intertidal species inhabiting Galician rocky shores are exposed to a strong vertical gra- dient in environmental and ecological conditions. Such a gradient is determined by a combination of wave action (impacting the low shore in particular) as well as heat, osmotic stress and predation (impacting the high shore in particular). In this area, 2 convergent shell polymorphisms have previ- ously been described in 2 marine gastropod species with direct development, Littorina saxatilis and Nucella lapillus. These distinct morphotypes occur sympatrically at the same shore at different levels, as a result of environmental adaptation. In the present work, we combine geometric morphometrics with the study of amplified fragment length polymorphism (AFLP) variation to investigate whether a similar polymorphism is occurring in individuals of the planktotrophic gastropod species Melarhaphe neritoides across this geographical region. Although no consistent genetic differentiation was found for AFLP, 2 distinct ecomorphs, similar to those already described in the other 2 species, were detected by morphometric analysis. Analyses of variance supported significant variation between morphs in 3 separate localities. The 2 morphs maintained most of their shell shape (and behavioural) differences when reared in laboratory conditions. These results suggest an adaptive role for the described polymorphism in size and shell shape although further studies are required to identify the genetic and evolutionary mechanisms favouring the maintenance of this pattern in sympatry in a high dispersal species.