Award: OCE-1130541

The deep sea (below 200m) is by far the largest ecosystem on the planet covering more than two-thirds of the surface. It supports a highly diverse and largely endemic fauna, yet virtually nothing is known about how all these species formed. Species formation requires the isolation of gene pools, but few obvious barriers exist in the deep sea that would impede gene flow and allow new species to form. The high diversity, lack of obvious isolating barriers, and broad-scale distribution of many taxa raise intriguing questions about how and where new species form in the deep sea. Our research represents the first concerted effort to study the genetic basis of how populations diverge in this vast ecosystem. We quantified the population genetic structure of several molluscan species arrayed along a depth gradient in the western North Atlantic. Genetic divergence among populations decreased with depth suggesting that the potential for population differentiation and speciation varied bathymetrically. Depth differences were considerably more important in fostering genetic divergence than geographic distances at the same depth. Patterns of genetic variation also indicated that deep-sea macrofauna could have strong population structure over small spatial scales, despite the lack of obvious isolating barriers. The small-scale divergence was often associated with depth differences and likely reflected the strong environmental gradients that attend changes in depth. Genetic divergence was sufficiently large for some species that they may represent cryptic species complexes. The presence of cryptic species suggests that geographic distributions may be greatly overestimated and biodiversity underestimated, which will have important implications for identifying the ecological forces that shape local and regional levels of diversity, understanding the evolutionary processes that promote diversification, and protecting the ecosystem properties essential for managing and preserving the deep-water fauna. These emerging phylogeographic patterns suggest that the environmental gradients paralleling changes in depth likely play an important role in the formation of new species in deep-water ecosystems and in the genesis and adaptive radiation of the deep-water fauna. Our research is providing the genetic tools to explore population structure in the deep sea, and producing the first critical evidence of how and where evolutionary differentiation occurs in this vast, remote and complex ecosystem. Unraveling how and where evolution unfolds is critical for explaining biogeographic patterns of diversity, predicting how deep-sea ecosystems might respond to climate change, developing conservation and management strategies to mitigate the intense exploitation of deep-sea resources and identifying appropriate locations and scales for MPAs. Broader Impacts This work has fostered a collaboration among biologists (UMB), and physical and paleo oceanographers (at WHOI) to better understand larval dispersal and connectivity in the deep ocean. In addition to our genetic work, we simulated larval dispersal at various depths in the western North Atlantic to test hypotheses about population connectivity and the potential role of the Deep Western Boundary Current (DWBC) in impeding gene flow between depths. Larval dispersal was estimated from Lagrangian particle trajectories simulated based on a deep-ocean circulation model (FLAME). Larval dispersal appeared to be quite likely between upper and lower bathyal populations because of strong across-slope transport mechanisms. This indicated that the DWBC was unlikely to preclude larval exchange between depth regimes and was unlikely to be responsible for the strong genetic divergence we typically find between upper and lower bathyal depths. This work also provided the first estimates of the nature and scale of larval dispersal at bathyal depths in the deep ocean. A number of students and postdocs have been involved...