Vertical distributions of invertebrate larvae in the deep-sea water columns of the Bahamas, the Gulf of Mexico and the southern Caribbean Sea

With the major exception of peracarid crustaceans, a large proportion of deep-sea animals produce planktonic larvae. The distribution of these larvae has important ramifications for population biology, genetic exchange and biogeographic patterns. Most information on larval densities in the deep sea comes from hydrothermal vent systems; the deep water columns overlying other deep-sea environments have seldom been sampled for larvae. We assessed larval distributions with MOCNESS tows over cold seeps in the Gulf of Mexico and the Barbados Accretionary Prism, and over island slopes in the Northern Bahamas, taking samples as deep as 4500m over the Barbados seeps. Most samples were sorted in their entirety prior to preservation, photographing and fixing larval morphotypes for later analysis. Larvae in most phyla were more abundant, often by several orders of magnitude, above the permanent thermocline. The number of larvae declined substantially as a function of depth, though samples very near the bottom were not taken. These results are consistent with at least three potential reproductive strategies: 1. larvae of deep-sea organisms spend minimal time suspended in the water column and do not disperse great distances, 2. larvae of deep-sea organisms pass through the water column quickly, dispersing with shallow-water larvae in the upper water column, and larvae of deep-sea animals disperse primarily near the bottom. Larval rearing studies combined with genetic analyses will be required to determine which of these applies to individual species.