Serpentinite-hosted chemosynthetic community of South Chamorro Seamount, Mariana Forearc

Deep-sea chemosynthetic ecosystems are ‘oases’ of life powered by reducing geofluids, of which serpentinite-hosted seeps are among the least studied. South Chamorro Seamount, a serpentine mud volcano on the Mariana Arc, has been known to host chemosynthesis-based assemblages since 1996, but no detailed information on the fauna was published. Here, we revisited South Chamorro to characterise its biodiversity. We located two regions of chemosynthetic communities dominated by bathymodioline mussels, vesicomyid clams, and chaetopterid parchment worms: one on the northwestern flank (‘Fryer Site’) and one on the southern summit (‘Summit Site’). We sampled a total of 20 species including 13 molluscs, five annelids, and two crustaceans – all present on the more active Summit Site but only a subset being found at Fryer Site. A mussel bed surrounding the Fryer Site was drilled by the Ocean Drilling Program in 2001 resulting in six holes, the deepest being 266 m (Hole 1200C). Cuttings ~50 cm deep still cover an approximately 60 m radius around Hole 1200C even 22 years later, and there is no sign of recovery. Low geofluid supply in serpentinite-hosted seamounts may not allow decadal recovery of animal colonies, unlike a previous drilling site in an Okinawa Trough vent.