Population differentiation in the open sea: insights from the pelagic copepod Pleuromamma xiphias.

Although a number of recent studies of marine holoplankton have reported significant genetic structure among populations, little is currently known about the biological and oceanographic processes that influence population connectivity in oceanic plankton. In order to examine how depth preferences influence dispersal in oceanic plankton, I characterized the genetic structure of a copepod with diel vertical migration (DVM) (Pleuromamma xiphias), throughout its global distribution, and compared these results to those expected given the interaction of this species' habitat depth with ocean circulation and bathymetry. Mitochondrial COI sequences from 651 individuals from 28 sites in the Indian, Pacific, and Atlantic Oceans revealed highly significant genetic differentiation both within and among ocean basins. Limited dispersal among distinct pelagic provinces seems to have played a major role in population differentiation in this species, with strong genetic breaks observed across known oceanographic fronts or current systems in all three ocean basins. The Indo-West Pacific (IWP) holds a highly distinct genetic population of this species that was sampled in both the western Pacific and eastern Indian Oceans. This suggests that the IWP does not act as a strong barrier to gene flow between basins, as expected, despite the relatively shallow water depth (<200 m) and vertically extensive (>400 m) diel migration of this species. A pattern of isolation by distance was observed in the Indian Ocean with genetic differentiation among samples down to spatial scales of ∼800 km, indicating that realized dispersal in P. xiphias occurs over much smaller spatial scales than in previously reported oceanic holoplankton. Given its highly regionalized population genetic structure, P. xiphias may have some capacity to adapt to local oceanographic conditions, and it should not be assumed that populations of this species in distinct pelagic biomes will respond in the same way to shared physical or climatic forcing.