The 'Expansion-Contraction' model of Pleistocene biogeography: rocky shores suffer a sea change?

Approximately 20,000 years ago the last glacial maximum (LGM) radically altered the distributions of many Northern Hemisphere terrestrial organisms. Fewer studies describing the biogeographic responses of marine species to the LGM have been conducted, but existing genetic data from coastal marine species indicate that fewer taxa show clear signatures of post-LGM recolonization. We have assembled a mitochondrial DNA (mtDNA) data set for 14 co-distributed northeastern Pacific rocky-shore species from four phyla by combining new sequences from ten species with previously published sequences from eight species. Nuclear sequences from four species were retrieved from GenBank, plus we gathered new elongation factor 1-alpha sequences from the barnacle Balanus glandula. Results from demographic analyses of mtDNA for five (36%) species (Evasterias troschelii, Pisaster ochraceus, Littorina sitkana, L. scutulata, Xiphister mucosus) were consistent with large population expansions occurring near the LGM, a pattern expected if these species recently recolonized the region. However, seven (50%) species (Mytilus trossulus, M. californianus, B. glandula, S. cariosus, Patiria miniata, Katharina tunicata, X. atropurpureus) exhibited histories consistent with long-term stability in effective population size, a pattern indicative of regional persistence during the LGM. Two species of Nucella with significant mtDNA genetic structure showed spatially variable demographic histories. Multilocus analyses for five species were largely consistent with mtDNA: the majority of multilocus interpopulation divergence times significantly exceeded the LGM. Our results indicate that the LGM did not extirpate the majority of species in the northeastern Pacific; instead, regional persistence during the LGM appears a common biogeographic history for rocky-shore organisms in this region.