Chemosynthetic mussels at a brine-filled pockmark in the northern Gulf of Mexico

A larg (540 square meters) bed of Bathymodiolus n. sp. (Mytilidae: Bivalvia) rings a pool of hypersaline (121.35 practical salinity units) brine at a water depth of 650 meters on the continental slope south of Louisiana. The anoxic brine (dissolved oxygen less than or equal to 0.17 milliliters per liter) contains high concentrations of methane, which nourishes methanotrophic symbionts in the mussels. The brine, which originates from a saltcored diapir that penetrates to within 500 meters of the sea floor, fills a depression that was evidently excavated by escaping gas. The spatial continuity of the mussel bed indicates that the brine level has remained fairly constant; however, demographic differences between the inner and outer parts of the bed record small fluctuations.