Deep-sea communities dominated by the giant clam, Calyptogena soyoae, along the slope foot of Hatsushima Island, Sagami Bay, Central Japan
Abstract Calyptogena -vestimentiferan tube worm communities were observed over a distance of roughly 7 km along the foot of the Hatsushima escarpment, Sagami Bay, central Japan, between depths of 900 and 1200 m by means of towed TV-still camera system and a submersible. The composition of the communities, the occurrence of a positive temperature anomaly of about 0.5°C within the community beds and an extremely high amounts of methane in the bottom water, indicated the presence of cold seepage. The biomass of living Calyptogena was estimated to surpass 10 kg m −2 , which was three orders of magnitude higher than that of the usual nearshore deep-sea benthic communities at an equivalent depth. The present results, combined with the electron microscopic evidence for symbiotic bacteria and the high sulfur content and negative σ 34 S value in the gills of the Calyptogena soyoae collected from the communities, and the stable and radioactive carbon isotopic compositions of the clam suggested that the seepage system of the Hatsushima site is similar to those found at other plate convergent areas such as off Oregon and off Peru and in the Japan Trench with symbiotic bacteria (probably sulfur and/or methane bacteria) playing important nutritional roles to support the seepage communities.