Oceanographic currents, geographic patterns and local environment contribute to neutral and adaptive genetic structure in two intertidal marine gastropods with contrasting life-histories

Global climate change is exposing intertidal organisms to increasing air and sea temperatures and changing ocean currents, affecting their ability to disperse, survive and reproduce, and resulting in shifts in their distribution and abundance. Improved understanding of these shifts requires characterization of population structure and local adaptation. We estimate the drivers of population structure in two intertidal gastropod species with contrasting life histories by assessing neutral and adaptive population structure and performing redundancy analyses in a seascape genomics framework. We show putative adaptive divergence between populations of the broadcast spawning topshell, Steromphala umbilicalis, despite high rates of neutral gene flow. This adaptive structure was best explained by geographic structure, separating sites in Wales from all other British and Irish sites. Larval dispersal, estimated from biophysical models, was also identified as a minor component explaining genetic connectivity in this species. For the direct developing dogwhelk, Nucella lapillus¸ neutral population structure was best explained by air and sea surface temperatures while putative adaptive population structure showed a greater influence of wave exposure. These results support the expected relationship between neutral population structure and reproductive mode (i.e., greater population structure in the direct developing N. lapillus compared with a lack of structure in the broadcast spawning S. umbilicalis) and highlight the interactive effects of geographic structure, larval dispersal and local environment on gene flow and adaptation of intertidal marine organisms.