In this study, we report experimental evidence of the thioautotrophic activity of the epibiotic microbial community associated with the setae of Shinkaia crosnieri, a galatheid crab that is endemic to deep-sea hydrothermal systems in the Okinawa Trough in Japan. Microbial consumption of reduced sulfur compounds under in situ hydrostatic and atmospheric pressure provided evidence of sulfur-oxidizing activity by the epibiotic microbial community; the rate of sulfur oxidation was similar under in situ and decompressed conditions. Results of the microbial consumption of reduced sulfur compounds and tracer experiments using (13)C-labeled bicarbonate in the presence and absence of thiosulfate (used as a thioautotrophic substrate) convincingly demonstrated that the epibiotic microbial community on S. crosnieri drove primary production via an energy metabolism that was coupled with the oxidation of reductive sulfur compounds. A combination of tracer experiments, fluorescence in situ hybridization (FISH) and nano-scale secondary ion mass spectrometry (Nano-SIMS) indicated that the filamentous cells of the genus Sulfurovum belonging to the class Epsilonproteobacteria were thioautotrophs in the epibiotic community of S. crosnieri. In conclusion, our results strongly suggest that thioautotrophic production by Sulfurovum members present as the epibiotic microbial community play a predominant role in a probable nutritional ectosymbiosis with S. crosnieri.