Limited Dispersal Drives Strong Genetic Structure in the Commercially Harvested Gastropod <i>Buccinum undatum</i> in the Western North Atlantic

Direct-developing species lack the pelagic larval phase which facilitates connectivity in most marine species. Consequently, they tend to exhibit spatially restricted dispersal and increased population structure. When subject to harvesting, this biological constraint increases their vulnerability to localized depletion, as local aggregations may be unable to recover through dispersal from neighboring areas. In eastern Canada, the direct-developing whelk <i>Buccinum undatum</i> is targeted by commercial fisheries. Declining landings and catch per unit effort have raised concerns that the species' fully benthic life history renders it vulnerable to localized overexploitation. Here, we leverage a large genome-wide dataset to elucidate patterns of spatial genetic structure in <i>B. undatum</i> and gain insight into how seascape features influence genetic connectivity. We sampled hundreds of individuals throughout Canadian northwest Atlantic waters and genotyped them at 23,405 SNPs. We detected five major genetic clusters, and considerable genetic substructure within most of these groupings. In the St. Lawrence Estuary, where geographic sampling was most intensive, isolation by distance, driven by limited dispersal along continuous habitat, was observed. Deep water also serves as a major barrier to gene flow, leading to genetic divergence among populations separated by less than 50 km. Exploratory analyses also indicate the potential for isolation by environment across the seascape. Overall, our results confirm the limited vagility and gene flow of <i>B. undatum</i>, which leads to hierarchical genetic structure across the seascape. These findings highlight the importance of managing whelk populations at local scales to protect distinct conservation units and support sustainable harvesting.