Trophic ecology of massive shrimp aggregations at a Mid-Atlantic Ridge hydrothermal vent site

The source of nutrition of shrimp that form giant aggregations at Mid-Atlan tic Ridge hydrothermal vents was explored by a combination of different mol ecular techniques. These animals have been hypothesized either to graze on the free-living, surface microbial community or to feed off epibiotic bacte ria growing on their exoskeleton. Stable isotope compositions of potential food sources, consumer tissues, and gut contents suggest that the shrimp de rive most of their carbon and nitrogen from the epibionts. However, probing of nucleic acids extracted from shrimp guts points to the existence of a g ut microflora that is isotopically similar but genetically distinct from th e epibionts. Carbon dioxide fixation experiments were carried out to compar e the magnitude of potential primary productivity of the different nutritio nal sources of the shrimp. Relatively low and comparable rates were observe d for both the free-living community and the epibionts but activity associa ted with gut preparations was surprisingly high. These high CO2 fixation ra tes in the gut may be indicative of a nutritional relationship between the shrimp and their gut microflora. Overall, the data indicate a nutritional s ymbiosis of the shrimp with their epibionts and possibly also with a separa te gut microflora.