SOURCE OF ENERGY SUSTAINING THE CALYPTOGENA POPULATIONS FROM DEEP TRENCHES IN SUBDUCTION ZONES OFF JAPAN

Deep tow camera surveys during the Hakuho Maru cruise (KH-89-1) and Na utile dives during the Kaiko-Nankai cruise (November 1989) demonstrate the presence of dense animal communities at depths around 2000 and 38 00 m. The dominant organisms are vesicomyid bivalve molluscs, with two new very large (up to 28 cm long) species of Calyptogena and the two species previously found in the Nankai Trough during the Kaiko cruise (1985), C. laubieri and C. kaikoi. They apparently rely on sulfide-bas ed chemoautotrophy through symbiotic bacteria associated with their gi lls. Evidence of sulfur-oxidizing metabolism includes ultrastructural features of symbionts, absence of methanol dehydrogenase activity, pre sence of ATP-sulfurylase and abundant elemental sulfur in the gill. Ca rbon isotope ratios are close to values obtained in other sulfur-oxidi zing symbiont-bearing species (from -35.6 to -38.7 parts per thousand for the 3950 m species and from -36 to -37.4 parts per thousand for th e 2050 m species). Nitrogen isotope ratios show highly variable values (from -4 to -9.7 parts per thousand for the 3950 m species and from - 0.2 to +4.4 parts per thousand for the 2050 m species). C-14 analyses indicates growth based on water CO2 with limited or no input of fossil carbon.