ABSTRACT: A dense community of benthic animals was discovered by the Japanese ROV 'Kaiko' in the hadal zone near the bottom of the Japan Trench, 7326 m deep. The community was dominated by a new species of thyasirid bivalve Maorithyas hadalis. This community appears to be sustained by chemosynthesis (nutrients being derived from reduced compounds within the sediment) for reasons including: a high concentration of sulfur contained in the thyasirid gills; the existence of numerous bacteria-like particles in the gill tissues; a sulfide smell from soft body parts and from collected sediment; the anoxic nature and high sulfide content of the sediments suggested by the dark gray to black color; the occurrence above geologic faults. Thyasirids have also been collected from other chemosynthesis-based communities around Japan, including the Japan Trench, Sagami Bay and the Nankai accretionary prism. Until now, the deepest chemosynthesis-based community known occurred on the Sanriku Escarpment of the Japan Trench, 6437 m deep. This site was dominated by the vesicomyid clam Calyptogena phaseoliformis. The discovery of a chemosynthesis-based community dominated by thyasirid clams from even deeper waters suggests a wider variety of chemosynthesis-based communities exists throughout deep-sea trenches. Additionally, the thyasirid species discussed here differs from other thyasirids by having circular symbionts which appear to exist intracellularly, and by living exposed above the sediment surface.

KEY WORDS: Chemosynthesis-based community · Thyasirid clams · Hadal zone · Japan Trench

Published in MEPS Vol. 190 (1999) on December 14
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1999