Phenotypic evidence for local adaptation to heat stress in the marine snail Chlorostoma (formerly Tegula) funebralis

Southern California (USA) populations of the intertidal marine snail Chlorostoma (formerly Tegula) funebralis generally occupy warmer climates and are exposed to high air temperatures during low tides more often than northern California populations. Available genetic data suggest there is extensive gene flow across a broad range of C. funebralis populations, so it is unclear if populations can adapt to differences in local environments. To test for population-specific responses to heat stress, three phenotypic assays were performed on three northern and on three southern populations of C. funebralis, after acclimation to common-garden conditions in the laboratory. Thermal drop-down, heat stress mortality, and heat stress reattachment assays were designed to evaluate ecologically relevant phenotypic responses to heat stress; these assays assessed tolerance during, mortality following, and speed of recovery following heat stress. The latter two tests indicate that southern populations consistently suffer significantly lower mortality and recover significantly more quickly following heat stress compared to northern populations. Hierarchical cluster analysis of stress response data clearly identified northern California and southern California regional groupings of populations. Thus, these results indicate that southern populations have higher tolerance to heat stress than northern populations and suggest that adaptation to local environmental differences can evolve despite moderate potential for larval dispersal in this species. Accounting for intraspecific population variation in thermal tolerance may provide important insights for predicting how species distributions will respond to global warming.