Community structure of mussel beds at deep-sea hydrothermal vents

Definition of biogeographic provinces, patterns of species distributions on local and regional scales, species richness, and relative abundances are all basic ecological measures, yet they are largely unknown for deep-sea hydrothermal vent ecosystems. Without an appreciation of biogeographic and biodiversity patterns, it is difficult to understand the evolutionary and ecological processes that underlie species distributions. To begin to address these issues, species composition, species richness, diversity, abundance, and recruitment patterns were studied within mussel beds at 3 remote hydrothermal fields on the southern East Pacific Rise (SEPR). Two closely spaced fields (within 1 km) were hydrothermally active and shared the most abundant taxa. The mussel bed communities at these fields could be differentiated primarily by the relative abundances of species rather than by species composition. A third field was in a waning stage of hydrothermal activity. Productivity, biomass, and abundance of invertebrates associated with mussel beds at this third field were low, and the species list was a small subset of that found at the active fields, subsidized by several nonvent deep-sea taxa. Based on abundances of smallest size classes, recruitment of mussels and of several other dominant invertebrate species was high at the active sites. At the waning vent field, no recruitment was observed, except within the population of ophiuroids. Most of the species found within the SEPR mussel beds belong to species previously described from vent sites north of the equator, indicating that there is a single hydrothermal biogeographic province extending along the East Pacific Rise for more than 30 degrees of latitude. There were no shared species between SEPR mussel beds and a Mid-Atlantic Ridge (MAR) vent mussel bed and species richness was more than twice as high at the active SEPR mussel beds than at the Mid-Atlantic Ridge mussel beds. As biogeographic provinces are defined and robust measures of species diversity accumulate for vent habitats on the East Pacific Rise and Mid-Atlantic Ridge, vent ecologists will be able to place the biogeography and biodiversity of vent faunas within the context of the regional and global biogeographic and diversity patterns observed in terrestrial and other marine ecosystems.