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 submer sible Nautile were devoted to the study of hydrothermal vent biology i n spreading centers of two Southwestern Pacific back-arc basins (Lau B asin and North Fiji Basin). In both cases, two major active sites were visited: White Lady and Mussel Valley in the North Fiji Basin and Hin e Hina and Vai Lili in Lau Basin. The faunal associations clustered ar ound active vents are dominated by two species of snails Ifremeria nau tilei and Alviniconcha hessleri and one or two species of mytilids bel onging to Bathymodiolus. These species are associated with chemoautoli thotrophic bacteria in intracellular symbiosis as detected by the acti vity of the Calvin-Benson cycle diagnostic enzyme RuBPcase. Pedunculat e and sessile barnacles dominated the outer rim of the site and are an alogs of the fiter-feeding serpulids living in the EPR sites. The hot extremes of the sites are poorly or not colonized by alvinellids or ot her taxa. In the Lau Basin, ''cold seep'' sites are found at the perip hery 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 i n side the ''Mussel Valley'' site.