A neutral DNA marker suggests that parallel physiological adaptations to open shore and salt marsh habitats have evolved more than once within two different species of gastropods

Local adaptation is an important mechanism generating physiological diversity and can be especially pronounced in species with restricted dispersal and gene flow such as direct developing snails of the genus Littorina. We compared physiological responses to salinity and desiccation stress in two co-occurring species of northeastern Pacific Littorina (L. subrotundata and L. sitkana) with salt marsh and open shore ecotypes. The animals from salt marsh populations were significantly more tolerant to low salinities and significantly less resistant to desiccation stress than their open shore counterparts. The lower resistance to desiccation in salt marsh animals was not associated with a higher rate of water loss during air exposure or with lower body water reserves, but instead reflected a lower tolerance to high salinities. These habitat-related physiological differences occurred in parallel in the two studied species of Littorina and persisted after prolonged laboratory acclimation, suggesting that they may reflect selection for markedly different local optima in the salt marsh habitats than in the open shore habitats. We used a neutral polymorphic nuclear DNA marker (intron of aminopeptidase N) to estimate the level of gene flow between the populations from different habitats and found isolation by distance regardless of the habitat from which the snails were collected. Our molecular data suggest that physiological cohesiveness of ecotypes can arise despite different genetic backgrounds, and could potentially be due to parallel evolution of convergent phenotypes in similar habitats.Communicated by O. Kinne, Oldendorf/Luhe