Submersible observations of the hydrothermal vent communities on the Iheya ridge, mid Okinawa trough, Japan

Abstract

During the Dives Nos. 409, 410, 480 and 481 of the Japanese submersible Shinkai 2000, conducted on June 10 and 11, 1989 and on May 16 and 17, 1990, several hydrothermal vents and prosperous vent associated biological communities were found on the northern slope of the Iheya Ridge in the Mid-Okinawa Trough (27°32.5′ N, 126°58.5′ E: depth 1,400 m). The first site we found, the Calyptogena Site, was characterized by a relatively thick blanket of sediments, pleated and/or laminated lava flows, with occasional lobate pillows and white and yellow stains. Although no remarkable shimmering water and thermal anomalies were detected during the observations, the extraordinarily dense community must be related to hydrothermal activities. The community was dominated by the giant white clam, Calyptogena okutanii, in biomass, and by Neolepas-type primitive scalpellids and slender vestimentiferan tube-worms in number. The second site, the Pyramid Site, situated only 200 m north ward from the Calyptogena Site has typical clear smokers emitting hot water over 200°C, and is characterized by a hard substratum of volcanic rocks and hydrothermal slabs. No noteworthy succession was perceived at the Calyptogena Site over a year. Many of the vent members occurred at both sites. However, Calyptogena okutanii, which were confined to te sediment bottom, Neolepas spp. and larger vestimentiferan tube-worms were found to thrive only at the Calyptogena Site, being only minor elements in the Pyramid Site. The global distribution of several groups of organisms is discussed preliminary in zoogeographical terms based on comparison with other submersible missions and surveys done by surface vessels.