Epifaunal community structure associated withRiftia pachyptilaaggregations in chemically different hydrothermal vent habitats

The vestimentiferan tubewormRiftia pachyptila(Polychaeta: Sibloglinidae) often dominates early succession stages and high productivity habitats at low-temperature hydrothermal vents on the East Pacific Rise. We collected 8 aggregations ofR. pachyptilaand the associated epifaunal community at 2 discrete sites of diffuse hydrothermal activity, in December 2001 and December 2002. Because of the high spatial and temporal variability of the biotic and abiotic factors related to hydrothermal vent activity, significant differences in the structure and the composition of the community were expected to occur at the scale of either 1 yr or 500 m distance between very different sites. There was no significant difference in the temperature ranges of the diffuse flow between sites or years, even though the environmental conditions were very different at the 2 sites. At 1 site (Riftia Field), the diffuse hydrothermal fluids had relatively low concentrations of sulfide, low pH, and high concentrations of iron. At the other site (Tica), the diffuse hydrothermal fluids had higher sulfide concentrations, the pH was closer to neutral, and iron was undetectable. The physiological condition ofR. pachyptilaappeared to reflect the availability of sulfide at each site. However, the structure and the composition of the epifaunal community were remarkably similar between sites and years, with the exception of a few species. Aggregations ofR. pachyptilasupport high local species diversity relative to the surrounding seafloor and high community similarity in different hydrothermal vent habitats.