Hierarchical genetic structure in a direct-developing whelk (Buccinum undatum) throughout Atlantic Canada

Direct-developing marine species lack a pelagic larval phase and are therefore predicted to have limited dispersal capacity and exhibit strong genetic structure over small spatial scales. But recent empirical work suggests that developmental mode, alone, cannot always explain realized connectivity patterns. We characterized spatial genetic structure within the western North Atlantic lineage of the waved whelk (Buccinum undatum), a direct-developing gastropod that is the target of an emerging fishery. We genotyped individuals throughout Atlantic Canada using 1,052 SNPs, documenting strong hierarchical genetic structuring throughout this region. Overall, pairwise genetic structure estimates were high, with two major genetic clusters separating southwestern and northeastern sites. But more complex patterns emerged at smaller spatial scales. Notably, pairs of sites on deeper offshore habitat exhibited panmixia, despite being separated by up to 100 km, whereas inshore and intertidal populations spaced at comparable distances were strongly genetically divergent. Collectively, our multi-scale analyses revealed that direct developers can exhibit genetic patterns that do not conform to simple geographic expectations. Future exploration of potential depth-variable connectivity is warranted, especially among harvested populations.