Species occurring in the mytilid and alvinellid communities within the 9°N hydrothermal field at the East Pacific Rise

Samples from mytilid mollusk and alvinellid polychaet communities and associated fauna were collected from the MIR-1 and MIR-2 manned submersibles during Cruise 49 of R/V Akademik Mstislav Keldysh at 9°? of the East Pacific Rise. Mytilids Bathymodiolus thermophylus occupied zones of diffuse discharge of hydrothermal solutions with temperature anomalies of some degrees. The alvinellid assemblages were located in the hottest parts of hydrothermal vents (~40°C) over sulfide mounds. Taxonomic compositions of these kinds of assemblages are different. Taxonomic richness of the mytilid communities is almost twice higher. Comparison of samples with use of the Jaccard's index showed greater similarity among alvinellid samples than that among mytilid ones. With respect to species number, polychaets formed the most diverse group, while gastropods were the second most diverse. Dominant species had greater proportions in the alvinellid communities than in mytilid ones. This study showed great differences between the mytilid and alvinellid associations in their species composition and community structure.