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The Håkon Mosby mud volcano (the Norwegian Sea, depth c. 1250 m) was studied in July 1998 by a joint Russian–German–USA–Norwegian expedition on the 40th cruise of the RV Akademik Mstislav Keldysh using the two Mir submersibles. The benthic community is dominated by two species of symbiotrophic pogonophores, Sclerolinum contortum (more abundant) and Oligobrachia haakonmosbiensis. The biomass of S. contortum reaches at least 435 g m ; for O. haakonmosbiensis the value is 350 g m . Different benthic organisms form associations with each species of pogonophore. The total list of known benthic fauna includes 46 species. A zoarcid fish, Lycodes squamiventer, is a common member of the benthic community. Bacterial mats are found over an extensive part of the crater. The background benthic community is much poorer and is dominated by ophiuroids (Ophiocten gracilis, Ophiopleura …