The intertidal zone represents an interesting model system to study adaptation because several ecological factors change abruptly across this area, causing strong divergent selective pressures on a scale of meters. On the Galician shore (NW Spain), 3 species (Littorina saxatilis, Nucella lapillus and Melarhaphe neritoides) show similar adaptation to survive these abrupt changes. The 3 species have independently evolved morphotypes with similar morphological features that occur either in sheltered or exposed environments. However, while in L. saxatilis and N. lapillus the morphotypes are genetically based, no genetic divergence has been found between the morphotypes of M. neritoides. To better understand the extent of divergence between the 2 morphotypes of M. neritoides and the causes of this polymorphism, protein expression patterns of morphotypes collected from 2 Galician localities were studied. Additionally, mtDNA genetic diversity and shell morphometrics were studied for a reference population at Silleiro. A significant and geographically consistent divergence in protein expression was found between morphotypes kept under controlled laboratory conditions for 1 mo. Morphometric analyses showed that the morphotypes maintained the differences in shell shape even after being kept for 15 mo under similar laboratory conditions. No significant genetic differentiation was found between the 2 morphotypes at the mtDNA level despite the high genetic diversity observed. Together, our data suggest phenotypic plasticity as the most likely cause for the morphotypes and provide putative candidate loci that can be further studied for a better understanding of the origin of this adaptive polymorphism.

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