Feeding specialization of bresiliid shrimps in the TAG site hydrothermal community

It is argued that the Atlantic bresiliid shrimps Rimicaris exoculata and Chorocaris chacei from a hydrothermal community on the Mid-Atlantic h d g e feed on exosymbiotic bacteiia that form a dense covenng on their mouthparts and the inner surface of the carapace. The associated bacterial community includes different morphological forms and types and is full of sulfide crystals from black smokers. Mouthparts of Atlantic hydrothermal bresiliids are hypertrophied and densely covered with setae to enlarge the attachment surface for associated bacteria. SEM revealed 2 dormnant morphological types of filamentous bactena with trichome width 0.2 to 0.5 pm and 1 to 3 pm, with elemental sulfur inside the cells and with attaching disks; both forms resemble Thlothriv spp. The idea of a bacterial source of food for shrimp was strengthened by carbon stable-isotope analysis, which yielded FI3C = 1 0 . 5 to -12.5%. Presumably, the shnmps keep close to hydrothermal fluid to farm their bacterial symbionts in the moderate current with a low concentration of hydrogen sulfide. The assayed level of ribulose 1,5-bisphosphate carboxylase activity in the bacterial community is low (2.2 pm01 min.' g-' protein), indicating dom~nance of heterotrophic forms among symbionts. The source of pnmary production in the vent community is discussed.