MICRODISTRIBUTION PATTERN AND BIOGEOGRAPHY OF THE HYDROTHERMAL VENT COMMUNITIES OF THE MINAMI-ENSEI KNOLL IN THE MID-OKINAWA TROUGH, WESTERN PACIFIC

From 1988 to 1992, a series of deep-sea surveys was conducted to characterize hydrothermal vent fields on the Minami-Ensei Knell, approximately 140 km west of Amami-Ohshima Island, southwest Japan, with a multi-narrow beam mapping system (Sea Beam), deep tow observing systems and the submersible Shinkai 2000. The vent fields were centered around the depressions on the western slope of the knell. The hydrothermal vents emitted superheated water over 269 degrees C through chimneys. Diffuse fluid discharged from fissures in rocks. Numerous patches of grayish white hydrothermal stains were observed on the bottom of coarse sand. Vent-associated biological communities consisted of sponges, vestimentiferans, alvinellid and polynoid polychaetes, cerithiid and trochid gastropods, lepetrodrillid limpets, vesicomyid clams, mytilid bivalves, bresiliid and hippolytid shrimp, zoarcid and cynoglossid fish, and lithodid and galatheid crabs. The hydrothermal vent communities of the Minami-Ensei Knoll showed many similarities to those of the Kaikata Seamount, the Mariana Back-Are Spreading Center, the North Fiji Basin and the Lau Basin, as well as the cold seep communities of Sagami Bay. There may be considerable interchange among the Minami-Ensei Knoll communities and other chemosynthetic communities in the Western Pacific despite the 1000 km distance separating these communities and the existence of Ryukyu Trench and Ryukyu Are. These discoveries, as well as other more recent findings around Japan, contribute significantly to our understanding of the biogeography of the hydrothermal vent and cold seep communities in the Western Pacific.