Evidence for genotypic differentiation between marine snails (Littorina sitkana) from the upper- and lower-intertidal zone in Bamfield Inlet (British Columbia, Canada)

Along wave-protected shores of the northeastern Pacific, the demography and life history of Littorina sitkana vary with intertidal zone, producing spatially close, but separate, “populations” (high- and low-shore). Field sampling (at multiple sites in the wave-sheltered Bamfield Inlet, B.C., Canada), a common garden experiment, and genetic analysis were used to determine if these two “populations” should be considered a panmictic population or different locally adapted populations (ecotypes). In the field, shell mass and incidence of white shells were greater for low-shore than high-shore snails, complementing other previously documented phenotypic differences (growth rate, size at sexual maturity). The common garden experiment revealed patterns of genetically-based phenotypic variation between the populations. Genetic analysis using a mitochondrial marker (COI) and a nuclear maker (APN54) did not resolve any spatial population genetic structure, but did confirm the absence of cryptic species. In the context of work on another littorinid (Littorina saxatilis), our study points to L. sitkana being an interesting species for further investigation of genetic differentiation between different populations depending on their position in the intertidal shore ecotone.