Bottom fauna associated with mussel beds and alvinellid communities in the hydrothermal field at 9° N of the East Pacific Rise

Samples from mytilid mollusk and alvinellid polychaet communities and associated fauna were collected during cruise 49 of R/V Akademik Mstislav Keldysh at 9° N of the East Pacific Rise. The mytilids Bathymodiolus thermophylus occupied zones of diffuse releases of hydrothermal solutions with temperature anomalies a few degrees in amplitude. The alvinellid assemblages were located in the hottest parts of hydrothermal vents (～ 40°C) over sulfide mounds. The taxonomic compositions of these kinds of assemblages are different. The taxonomic richness of mytilid communities is almost twice higher. A comparison of the samples using Jaccard’s index showed a greater similarity among the alvinellid samples than that between the mytilid ones. With respect to the species number, polychaets formed the most diverse group, while gastropods were the second most diverse. The dominant species had greater proportions in alvinellid communities than in mytilid ones. This study showed great differences between the mytilid and alvinellid associations in their species composition and community structure.