Meiofaunal diversity and assemblage structure in a shallow-water hydrothermal vent in the Pacific Ocean

Despite their ubiquitous distribution in tectonically active coastal zones, shallow-water vents have been much less explored than deep-sea vents in terms of biodiversity and adaptations to extreme conditions. We investigated the meiofaunal biodiversity and environmental variables at dis- tances of 10, 100 and 200 cm from a shallow-water hydrothermal vent in the equatorial Pacific (Sulawesi, Indonesia). Meiofaunal abundance and the richness of higher taxa increased from the site of fluid-vent emission (where temperatures of the fluids and sediments reached approximately 90°C) to the control sediments (200 cm from the vent, with no sign of effects from the vent fluids). Nema- tode species richness was also high in the intermediate station, where bottom sediment temperature reached 55°C. These data suggest that some nematode species were able to survive in conditions typically hostile to metazoan life. Gas emissions also influenced the biochemical composition of the sediment organic matter in proximity to the vent and favoured the growth of a large photo- and/or chemo-autotrophic prokaryotic biomass. This biomass represented a potentially important food source for predator/omnivore nematodes and influenced the trophic structure of benthic assem- blages. Since the metazoan species found in proximity to the vent were a subset of those inhabiting control sediments, but were characterised by lower abundances, it might be hypothesized that the populations close to the vent are the result of colonization from adjacent areas