Associations between methanotrophic bacteria and several species of mytilids and a pogonophore have been described in deep-sea communities surrounding hydrothermal vents and cold seeps. We report here a new symbiosis between a sponge and methanotrophic bacteria. The delta 13C values and distribution indicate that the sponge is nutritionally reliant on its methanotrophic symbionts. Ultrastructural evidence indicates intracellular digestion of the bacteria. Intracellular symbionts were found in brooded embryos, indicating direct transmission of the symbionts between generations. The sponge, Cladorhiza sp., belongs to a unique deep-sea family, Cladorhizidae (Demospongiae), which are carnivorous, and lack an aquiferous system. The specimens were collected from a mud volcano in the Barbados Trench at a depth of 4,943 m. Large aggregations of hundreds of individuals (up to 1.5 m in diameter and 40 cm high) were found around the periphery of the volcano's eye, where the highest concentrations of methane were documented. Before this discovery, cladorhizids were known to occur only in low densities as discrete individuals.
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