Different energy sources for three symbiont-dependent bivalve molluscs at the Logatchev hydrothermal site (Mid-Atlantic Ridge)

The vent mussel Bathymodiolus puteoserpentis, a large vesicomyid clam and a smaller thyasirid were collected from an area of sediment subject to diffu se hydrothermal flow. The mussels live on the surface, the vesicomyids are partly buried and the thyasirids burrow in the sediment. The fine structure of the gills differs in the three bivalves. Bathymodiolus puteoserpentis h osts two types of bacterial symbiont, one methanotrophic, and another proba bly thiotrophic. The other two bivalves have, single types of symbiont of d ifferent shapes. Stable isotope ratios of carbon and nitrogen indicate thio trophy in the vesicomyid and thyasirid, but a predominance of methanotrophy in the mussel. This is the first time that such an assemblage has been fou nd at a hydrothermal site on the Mid-Atlantic Ridge (MAR), with the differe nt faunistic elements exploiting different energy resources.