Mitochondrial Spatial Diversity in a Keystone Salt Marsh Mussel Geukensia demissa

Marine ecosystems tend to display rapid species turnover as well as stark transitions in intraspecific genetic diversity across consistent “breakpoints” on the Atlantic coast of the United States, but this phenomenon has primarily been documented at low spatial resolution, especially with respect to keystone marsh species. The Atlantic ribbed mussel Geukensia demissa plays an important role in the health and productivity of salt marsh ecosystems; as such, describing the overall patterns of connectivity and isolation of spatial populations of G. demissa will contribute significantly to the broader understanding of marsh ecosystem dynamics and biogeographic turnover. Here, decreasing nucleotide diversities (π) for samples of G. demissa from south to north across its distributional range from southern Florida to Nova Scotia suggest a transition from high genomic diversity in the southern portion of the range to lower diversity, sometimes with private haplotypes marking spatial samples in the northern range. Repeated analyses of molecular variance (AMOVA) yield highly elevated regional partitioning of diversity when regions are divided by canonical biogeographic breakpoints, especially across the Bay of Fundy. These results, when viewed in the context of our current understanding of biogeography and glacial refugia, suggest high dispersal potential in G. demissa but also metapopulation dynamics of local extirpation and recolonization.