Deep-sea hydrozoans in the western part of the Bering Sea: some hydroids associated with methane seep communities at upper bathyal depths and abyssal hydroids with a description of two new species

Seven hydrozoan species were collected from the abyssal depths of the Volcanologists Massif and Komandorsky Basin (-3360-3930 m) and from methane seep communities on the Koryak slope at upper bathyal depths (-660 m) during the cruise of the RV Akademik M.A. Lavrentyev in 2018 in the western part of the Bering Sea. Hydroids were used for detailed morphological analysis and identified using mitochondrial (16S rRNA) and nuclear (18S, 28S) markers. Two new species of large hydroids, Branchiocerianthus radialis sp. nov. and Candelabrum beringensis sp. nov., were described from the abyssal depths. Branchiocerianthus radialis sp. nov. has an unusual radial symmetry, uncharacteristic for this genus. However, molecular phylogenetic data and dichotomous branched blastostyles unequivocally indicate its belonging to the genus Branchiocerianthus. Candelabrum beringensis sp. nov. differs from C. phrygium, widespread in the Northern Hemisphere, by the root-like foot zone covered by multiple tentacle-like suckers and rare specialized tentacles. Earleria cf. denticulata is most likely a wide spread species inhabiting bathyal and abyssal biotopes in the Pacific and Atlantic Oceans. Cryptic biodiversity was found for phylogenetic clade Lafoea cf. dumosa sp. 2, including two species from bathyal and abyssal depth of the Bering Sea. Methane seep communities were locations with enhanced diversity of hydroids and were inhabited by numerous so called "hydroid bushes" dominated by species L. cf. dumosa (presumably L. cf. dumosa/sp. 2C, Lafoeidae) and various sertulariids attached to carbonate crusts and pebbles. Four species from the methane seep communities were analyzed in detail and identified basing on molecular data: L. cf. dumosa/sp. 2C, Abietinaria cf. kinkaidi, Tubularia sp. and Hydrodendron cf. stechowi. Phylogenetic position of two species was unexpected: A. cf. kincaidi (Sertulariidae) showed no relationship with available Abietinaria spp. and grouped with sertulariid Thuiaria thuja, and H. cf. stechowi currently assigned to Phylactothecidae grouped with the species of the family Zygophylacidae. Our results confirm the presence of hard-to-identify hydrozoan species (Lafoea spp., Earleria spp., Candelabrum spp.) that differ at different depths. Therefore careful morphological and molecular phylo-genetic studies are needed for species identification of deep-sea specimens.