DEEP-SEA HYDROTHERMAL COMMUNITIES IN SOUTHWESTERN PACIFIC BACK-ARC BASINS (THE NORTH FIJI AND LAU BASINS) - COMPOSITION, MICRODISTRIBUTION AND FOOD-WEB

During the year 1989, two diving cruises of the French deep-sea submersible Nautile were devoted to the study of hydrothermal vent biology in spreading centers of two Southwestern Pacific back-arc basins (Lau Basin and North Fiji Basin). In both cases, two major active sites were visited: White Lady and Mussel Valley in the North Fiji Basin and Hine Hina and Vai Lili in Lau Basin. The faunal associations clustered around active vents are dominated by two species of snails Ifremeria nautilei and Alviniconcha hessleri and one or two species of mytilids belonging to Bathymodiolus. These species are associated with chemoautolithotrophic bacteria in intracellular symbiosis as detected by the activity of the Calvin-Benson cycle diagnostic enzyme RuBPcase. Pedunculate and sessile barnacles dominated the outer rim of the site and are analogs of the fiter-feeding serpulids living in the EPR sites. The hot extremes of the sites are poorly or not colonized by alvinellids or other taxa. In the Lau Basin, ''cold seep'' sites are found at the periphery of active hot or warm ventsand are dominated by vestimentiferans and pogonophorans. No major differences were seen between associations of the two back-arc basins at the generic level with the exeption of the abundance of synaptid holothurians associated with Bathymodiolus in side the ''Mussel Valley'' site.