Colonization and/or mitochondrial selective sweeps across the North Atlantic intertidal assemblage revealed by multi-taxa approximate Bayesian computation.

Intertidal and subtidal communities of the western and eastern coasts of the North Atlantic Ocean were greatly affected by Pleistocene glaciations, with some taxa persisting on both coasts, and others recolonizing after being extirpated on one coast during the Last Glacial Maximum. In the original spirit of comparative phylogeography, we conducted a comparative analysis using mtDNA sequence data and a hierarchical approximate Bayesian computation (ABC) approach for testing these two scenarios across 12 intertidal and subtidal coastal invertebrates spanning the North Atlantic to determine the temporal dynamics of species membership of these two ephemeral communities. Conditioning on a low gene-flow model, our results suggested that a colonization or mitochondrial selective sweep history was predominant across all taxa, with only the bivalve mollusc Mytilus edulis showing a history of trans-Atlantic persistence. Conditioning on a high gene-flow model weakened the support for this assemblage-level demographic history. The predominance of a colonization-type history also highlights concerns about analyses based on single-locus data where genetic hitchhiking may be incorrectly inferred as colonization. In conclusion, driving factors in shifting species range distributions and membership of ephemeral coastal communities could be species-specific environmental tolerances, species interactions, and/or stochastic demographic extinction. Through a re-examination of a long-standing question of North Atlantic phylogeography, we highlight the flexibility and statistical honesty of using a model-based ABC approach.