Hadal trenches, some of the Earth’s least explored and understood environments, have long been proposed to harbour chemosynthesis-based communities^1,2. Despite increasing attention, actual documentation of such communities has been exceptionally rare^3,4. Here we report the discovery of the deepest and the most extensive chemosynthesis-based communities known to exist on Earth during an expedition to the Kuril–Kamchatka Trench and the western Aleutian Trench using the manned submersible Fendouzhe. The communities dominated by siboglinid Polychaeta and Bivalvia span a distance of 2,500 km at depths from 5,800 m to 9,533 m. These communities are sustained by hydrogen sulfide-rich and methane-rich fluids that are transported along faults traversing deep sediment layers in trenches, where methane is produced microbially from deposited organic matter, as indicated by isotopic analysis. Given geological similarities with other hadal trenches, such chemosynthesis-based communities might be more widespread than previously anticipated. These findings challenge current models of life at extreme limits and carbon cycling in the deep ocean.

A new species of nephtyid polychaetes Nephtys sachalinensis sp. n. from the upper sublittoral zone (at a depth of 10–107 m) off Northeast Sakhalin, the Sea of Okhotsk, is described. Diagnostic characters of adults are: an everted pharynx with 20 bifid terminal and 2 median simple terminal papillae; single median dorsal and ventral pharyngeal subterminal papillae absent; proximal and median parts of the pharynx are covered with verrucae; branchiae begin from the fourth chaetiger; acicular lobes are distinctly bilobed from the anterior chaetigers almost to the end of the body; each acicular lobe (both in notoand neuropodia) has one rounded outgrowth on its interramal side; subulate notopodial cirri at the median and posterior chaetigers; neuropodial postacicular lobes are no more than two times longer than the acicular lobes. The new species is compared with its related species.