Découverte de communautés animales profondes durant la campagne franco-japonaise KAfKO de plongées dans les fosses de subduction autour-du Japon.

Twenty seven dives of the submersible Nautile in the subduction zones around Japan conducted during Summer 1985 in the French-Japanese Project KAfKO proved that fairly luxuriant benthic animal communities dominated by deep-sea giant clams belonging to the genus Calyptogena (family Vesicomyidae) wereconsistently present at abyssal depths,from 3,800 to 5,960m ([1], [2]). According to the bathymetric range and the geographic locations, two different types of clam colonies have been discovered: The jirst type ex tends from 3,800 to 4,020 m in the mouth of Tenryu canyon on the accretionary prism and at the top of basement swell of Zenisu ridge, both located in the eastern Nankai subduction zone. The bivalve colonies are composed of two different undescribed species of Calyptogena, often associated with several species of invertebrates and different types of animal tracks at the sediment surface. Recurrent dives on the same colony separated by four days enable to demonstrate that Calyptogena spp. are able to move around on several decimeters keeping their vertical position within the sediment with the hinged apex of the shell as leading edge - The second type extends from 5,130 to 5,960m in the landward wall of the Japan trench, the axis of the Japan trench West of Kashima seamount and the landward wall of western Kurile trench. This community is dominated by a third undescribed species of Calyptogena of peculiar slender shape frequently associated with several other invertebrate species which vary according to depth and sites. This type of colony has a less continuous distribution and a greater variability in colony size than the jirst type. The crude biomass (total wet weight including the shell) is extremely high for such depths, ranging from 16kgjm2 (Tenryu canyon and Zenisu ridge) to 24-42kgjm2 (near Kashima seamount) and even .51 kgjm2 (Japan trench), for Calyptogena only. Such high biomasses combined with the occurrence of haemoglobin in ail three Calyptogena species and the situation of the colonies in the geological framework clearly suggest that these communities are sustained by chemosynthetic processes rather than photosynthetic organic sources. Similar communities have been recently described at shallower depths on the accretionary prism of the Oregon subduction zone [3] and a model of chemosynthetic energy source derived from pore water methane has been proposed [4]. Geochemical analyses performed on water samples from Japan and Kurile trenches communities demonstrate the occurrence of seepage of pore water containing thermogenicmethane [5]. Although sorne work remains to be done, the distribution, disposition and densities of the japanese bivalve colonies can only be explained in the general context of chemosynthetic mechanisms using methane dissolved in pore water squeezed from the sediments as a chemical source of energy. These communities are to date the deepest record of benthic life associated with chemosynthetic processes.

Au cours de vingt-sept plongées du submersible Nautile pendant l'été 1985 dans les fosses de subduction autour du Japon (fosses de Nankai, du Japon et des Kouriles), des communautés benthiques animales caractérisées par la prédominance de grands bivalves du genre Calyptogena (famille des Vesicomyidae) ont été découvertes dans des sédiments abyssaux. Les communautés observées entre 3800 et 4020m dans le canyon de Tenryu situé sur le prisme d'accrétion sédimentaire de la fosse de Nankai et au pied de la ride de Zenisu, sont dominées par deux espèces distinctes de Calyptogena nouvelles pour la science. Des communautés souvent représentées par des colonies dispersées occupant des surfaces plus variables, caractérisées par une troisième espèce de Calyptogena également nouvelle, ont été observées plus profondément.entre 5130 et 5960m sur les pentes internes des fosses du Japon et des Kouriles. L'analyse des photographies prises par le Nautile permet de préciser l'importance de la biornasse brute de ces communautés, qui varie entre 16 et 51 kgjm2 (poids frais coquilles comprises). Plusieurs observations et la comparaison avec d'autres communautés animales profondes découvertes récemment indiquent que ces colonies de bivalves dépendent en majeure partie de phénomènes chimiosynthétiques eux-mêmes liés aux süintements d'eau interstitielle expulsée des sédiments profonds par la subduction. Le méthane, présent dans l'eau de mer qui baigne ces colonies, est vraisemblablement à l'origine de la chimiosynthèse. Jusqu'à présent, on ne connaissait pas de communautés chimiosynthétiques au-delà d'environ 3000 m.