Concordance between genetic diversity and marine biogeography in a highly mobile marine mammal, the Risso's dolphin

Aim: The heterogeneity of the marine environment is thought to be the origin of marine biodiversity, often delineated in marine biogeographical provinces. Cetaceans are highly mobile aquatic mammals, but even those species inhabiting seemingly boundary-free open waters are found to exhibit degrees of population structure, often attributed to ecological and behavioural factors such as resource specialization and site fidelity. Our aim in this study is to test the hypothesis that a cosmopolitan, resource-specialist dolphin species, the Risso's dolphin (Grampus griseus) will show an association between genetic diversity and biogeographical provinces. Location: North Pacific and North Atlantic Oceans. Methods: We examined genetic variation at 19 microsatellite loci among 236 samples collected from a range of locations in the North Pacific Ocean, and for a 473 bp mitochondrial DNA control region sequence in 213 samples from the North Pacific and North Atlantic Oceans to assess genetic diversity, population structure and historic population dynamics. Results: We found differentiation in the North Pacific primarily among three populations corresponding to the marine biogeographical provinces of the Kuroshio-Oyashio Current (Sino-Japanese), California Current (California), and Eastern Tropical Pacific. Analyses using mitochondrial DNA data further reveal that these three populations are genetically different from the populations in the North Atlantic Ocean and Mediterranean Sea, without strong lineage sorting, and that the western North Pacific population experienced a period of population expansion while the Eastern Tropical Pacific population remained stable. Main conclusions: We find evidence for biogeographical endemism in a highly mobile marine mammal species that nevertheless shows evidence for long-range dispersion within and to a lesser extent among biogeographical provinces. We conclude that distinct ecological processes promote differentiation, even though this species forages on narrow range of prey species.