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 Ridge feed on exosymbiotic bacteria that form a dense covering 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 dominant morphological types of filamentous bacteria with trichome width 0.2 to 0.5 mum and 1 to 3 mum, with elemental sulfur inside the cells and with attaching disks; both forms resemble Thiothrix spp. The idea of a bacterial source of food for shrimp was strengthened by carbon stable-isotope analysis, which yielded deltaC-13 = -10.5 to -12.5 parts per thousand. Presumably, the shrimps 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 mumol min-1 g-1 protein), indicating dominance of heterotrophic forms among symbionts. The source of primary production in the vent community is discussed.