Active positioning of vent larvae at a mid-ocean ridge

The vertical position of larvae of vent species above a mid-ocean ridge potentially has a strong effect on their dispersal. Larvae may be advected upward in the buoyant vent plume, or move as a consequence of their buoyancy or by active swimming. Alternatively, they may be retained near the bottom by the topography of the axial trough, or by downward swimming. At vents near 9°50′N on the axis of the East Pacific Rise, evidence for active larval positioning was detected in a comparison between field observations of larvae in the plankton in 2006 and 2007 and distributions of non-swimming larvae in a two-dimensional bio-physical model. In the field, few vent larvae were collected at the level of the neutrally buoyant plume (~75 m above the bottom); their relative abundances at that height were much lower than those of simulated larvae from a near-bottom release in the model. This discrepancy was observed for many vent species, particularly gastropods, suggesting that they may actively remain near the bottom by sinking or swimming downward. Near the seafloor, larval abundance decreased from the ridge axis to 1000 m off axis much more strongly in the observations than in the simulations, again pointing to behavior as a potential regulator of larval transport. We suspect that transport off axis was reduced by downward-moving behavior, which positioned larvae into locations where they were isolated from cross-ridge currents by seafloor topography, such as the walls of the axial valley—which are not resolved in the model. Cross-ridge gradients in larval abundance varied between gastropods and polychaetes, indicating that behavior may vary between taxonomic groups, and possibly between species. These results suggest that behaviorally mediated retention of vent larvae may be common, even for species that have a long planktonic larval duration and are capable of long-distance dispersal.