Marine Biology98, 209-216 (1988) Marine ................. Biology 9 Springer-Verlag 1988 Feeding biology of the shrimp Rimicaris exoculata at hydrothermal vents on the Mid-Atlantic Ridge C.L. Van Dover 1, B. Fry 2, J.F. Grassle 1, S. Humphris ~ and P.A. Rona ~ 1 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 2 Ecosystems Center, Marine Biological Laboratory,Woods Hole, Massachusetts 02543, USA 3 US Department of Commerce, NOAA/AOML, 4301 Rickenbacker Causeway, Miami, Florida 33149, USA Abstract A newly described species of shrimp, Rimicaris exoculata Williams and Rona, 1986, dominates the megafaunal community at two hydrothermal vent sites on the Mid- Atlantic Ridge. Behavioral observations and gut-content analyses indicate that these shrimp ingest large amounts of sulfide particles from black smoker chimneys. We found no evidence for chemoautotrophic endosymbionts in R. exoculata, based on analyses of morphology, stable isotopes, lipopolysaccharides, and ribulose-l,5-bisphos- phate carboxylase (RuBPCase) activity. Instead, we sug- gest that the shrimp are normal heterotrophs, grazing on free-living microorganisms associated with black smoker chimneys. We infer that high bacterial productivity is required to sustain populations of R. exoculata at these vent sites. Introduction Hydrothermal vent and cold-water seep communities in the deep sea have been characterized by an abundance of large, relatively sessile megafauna. Tubeworms, clams, and mussels that harbor symbiotic, chemoautolithotrophic bac- teria typically dominate these communities (reviewed by Grassle 1986). In contrast, tubeworms were not observed at newly discovered hydrothermal vent communities on the Mid-Atlantic Ridge (MAR) and bivalve molluscs are rare (Rona 1985, Detrick et al. 1986 a, b, Grassle et al. 1986, Rona et al. 1986). These sites, however, do possess a distinc- tive fauna dominated by active, motile swarms of caridean shrimp. The shrimp belong to a new genus, Rimicaris, and two new species, B. exoculata and R. chacei, in the family Bresiliidae (Williams and Rona 1986). Both Rimicaris exoeulata and R. chacei appear to be widely distributed along the Mid-Atlantic Ridge (MAR). They have been found at the TAG and Snake-Pit hydro- thermal sites (Williams 1987), areas separated by 307 krn and the Kane Fracture Zone. At these sites, R. exoculata was by far the more abundant species in the material collected. This shrimp, 4 to 5 cm in total length, has a distinctive morphology, with enlarged antennal and antennular peduncles, no rostrum, no lenses or eyestalks, and a carapace that wraps almost entirely around the animal, creating a nearly tubular branchial chamber on either side (Williams and Rona 1986). The gills of R. exo- culata are of typical external morphology, but the exopods of the first maxillipeds and the second maxillae are atypical, being enormously expanded (Williams and Rona 1986). Chemical analyses of black smoker waters indicate a qualitative similarity between MAR and East Pacific Rise hydrothermal effluents (Edmond etal. 1986), including elevated hydrogen sulfide concentrations. Previous studies of deep-sea vent communities suggest that their food webs are based on primary production by chemoautotrophic prokaryotes that use hydrogen sulfide as an energy source (Karl etal, 1980, Cavanaugh etal. 1981, Felbeck 1981). Chemoautotrophic production is likely to be important at MAR vents as well. We studied the trophic biology of Rimiearis exoculata to determine how chemoautotrophic production may be linked to high shrimp-population densities. Materials and methods Rimicaris exoeulata used in this study were collected in 1985 by dredging at the TAG hydrothermal field (26~ 44~ 3 630 m) in the rift valley of the Mid-Atlantic Ridge. A portion of the dredged material was preserved in formalin and stored in 70% EtOH; the remainder was frozen. Video records of shrimp behaviour in situ were made at the TAG and Snake-Pit (23~ 44~ 3 500 m) sites in 1986 during "Alvin" Dives 1675-1677 and 1683, respectively. Observations of shrimp at TAG and