Genomic separation of Salish Sea and Pacific outer coast populations of the keystone sea star Pisaster ochraceus.

Environmental boundaries shape genetic diversity through the interacting effects of geographic distance, local adaptation, and constraints on gene flow. The ochre sea star (Pisaster ochraceus), an intertidal keystone predator, has long been considered to have limited spatial genetic structure along the North American Pacific coast, likely due to its extended larval dispersal period and high potential for gene flow. Here, we characterize spatial genomic variation in Pisaster ochraceus using whole-genome sequencing data from individuals spanning nearly 3000 kilometers of coastline from Alaska to southern California. Analyses of putatively neutral SNPs demonstrate considerable mixing across the latitudinal range, but also reveal substantial structure between outer Pacific coast populations and those within the semi-enclosed Salish Sea, suggesting restricted gene flow and demographic divergence between these regions. Genomic divergence is further supported by evidence of selection, with outlier loci highlighting extended regions of low diversity in the Salish Sea, consistent with recent selective sweeps and potential local adaptation to distinct estuarine conditions. These findings support the role of oceanographic barriers and environmental heterogeneity in shaping population structure in Pisaster ochraceus, challenging earlier expectations of range-wide homogeneity and providing insight into the persistence of this keystone marine species in a rapidly changing world.