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Benthic communities of the hydrothermally active submarine Piip Volcano (55°445 N, 167°263 E, 368–495 m, the southwestern Bering Sea) were explored during the 75th and 82nd cruises of the RV Akademik M.A. Lavrentyev using the ROV Comanche 18. The Piip Volcano is the northernmost hydrothermal region in the Pacific. Macrofauna composition and community structure were investigated based on qualitative and relative abundance samples along with still images and video records. In total, 130 species of macro- and megafauna were found, of which ∼25% are new to science. Of the fauna, 9 species (7%) are obligate or potentially obligate to environments with access to reduced chemicals. These species are the bivalve Calyptogena pacifica (Vesicomyidae: Pliocardiinae), the gastropods Parvaplustrum wareni (Parvaplustridae) and Provanna sp. (Provannidae), the amphipod Onesimoides sp. (Tryphosidae), the leptostracan Nebalia piipensis (Nebaliidae), the isopod Eurycope andreyi (Munnopsidae), and the polychaetes Neosabellides uschakovi (Ampharetidae), Ophryotrocha beringiana (Dorvilleidae) and Harmothoe globosa (Polynoidae). The total species richness of the Northern and Southern summits was almost identical (90 species and 85 spp respectively), but the number of species on sites without bacterial mats on the Northern Summit (81 spp) was almost 1.5 times higher than on the Southern Summit (57 spp). At the same time the species richness was almost twice higher in habitats with bacterial mats and/or clam beds on the Southern Summit (58 spp) compared to the Northern one (32 spp). The main faunal difference was the absence of Calyptogena pacifica on the Northern Summit. The factors that may explain differences of the faunal communities of the Northern and Southern summits are discussed. The distinctive features of macro- and megafaunal communities of the Piip Volcano were the significant number of taxa, obligate for reducing habitats, of different taxonomic levels including family and subfamily levels and plentiful abundances of sedentary organisms, including the sea anemone Corallimorphus cf. pilatus , the demosponge Vulcanella koltuni , calcareous sponges and benthopelagic Trachimedusa, all of them presumably utilizing the rich food resources of reducing habitats. • 130 mega- and macrofaunal species were found on the hydrothermally active Piip Volcano at 368–495 m depth. • Among 9 species presumably obligate to reducing habitats, 5 were also recorded from seeps outside the volcano. • Numerous sedentary organisms found at bacterial mats, were also known from regular habitats outside the Piip Volcano. • Calyptogena beds were present only on the Southern Summit. • Species richness at bacterial mats and clam beds on the Southern Summit was twice as high as that of the Northern Summit. [ABSTRACT FROM AUTHOR]

Methane seep communities on the Koryak slope in the Bering Sea were discovered in 2018 during the 82nd cruise of the RV Akademik M.A. Lavrentyev using ROV Comanche 18. Methane seeps found between 400 m and 695 m depth are the northernmost chemosynthesis-based habitats (∼61°N) known to date in the Pacific. Macrofauna species composition and community structure of methane seep and background benthic communities were investigated at three depth horizons: 400–402 m, 417–429 m and 647–695 m. A total of 335 macrofaunal species were identified. At the depth 647–695 m the species richness in the methane seep communities was 265 species and 72 species in the background. The deepest seep communities were characterized by the highest species richness, relatively high evenness and strong variations in the composition and structure. At shallower depths the species richness was low: 33–34 species at methane seeps and 66–71 in the background. The methane seep communities at shallow depths showed relatively low evenness and low diversity of community structure. The highest species richness at the greatest depth can be related in part to increased habitat heterogeneity associated with carbonate crusts and vesicomyid clam beds occurring only at deeper methane seeps. The deepest methane seep communities were dominated by the chemosymbiotrophic pliocardiine Calyptogena pacifica ; dominant in the background was the ophiuroid Ophiophthalmus normani. At shallower methane seeps only single specimens of C. pacifica were found. The shallower methane seeps were dominated by the background species despite numerous extensive bacterial mats and high potential toxicity of the environment. The seep community at 417–429 m was dominated by the echinoid Brisaster latifrons with population density on the bacterial mat four times higher than in the background. Four chemosymbiotrophic species were recorded at 647–695 m and only one was found at shallower depths. Potentially obligate to reducing habitats polychaetes of the genus Neosabellides and Ophryotrocha occurred in methane seep communities at all depths. Further geochemical, geological and microbiological studies are required to better characterize differences of methane seeps at different depths and reasons of these differences. • Methane seeps were discovered on the Koryak slope (Bering Sea) at 400–700m depth. • Composition and structure of seep communities differed at depths 400–402m, 417–429m and 647–695m. • Background species, echinoids and actiniarians, dominated the shallower seeps. • Chemosymbiotrophic pliocardiine Calyptogena pacifica dominated the deepest seeps. • The deepest seeps have the highest species richness and vary in community structure. [ABSTRACT FROM AUTHOR]