A gastropod from a deep-sea hydrothermal field at the Rodriguez triple junction, Indian Ocean, has scale-shaped structures, called sclerites, mineralized with iron sulfides on its foot. No other organisms are known to produce a skeleton consisting of iron sulfides. To investigate whether iron sulfide mineralization is mediated by the gastropod for the function of the sclerites, we performed a detailed physical and chemical characterization. Nanostructural characterization of the iron sulfide sclerites reveals that the iron sulfide minerals pyrite (FeS 2 ) and greigite (Fe 3 S 4 ) form with unique crystal habits inside and outside of the organic matrix, respectively. The magnetic properties of the sclerites, which are mostly consistent with those predicted from their nanostructual features, are not optimized for magnetoreception and instead support use of the magnetic minerals as structural elements. The mechanical performance of the sclerites is superior to that of other biominerals used in the vent environment for predation as well as protection from predation. These characteristics, as well as the co-occurrence of brachyuran crabs, support the inference that the mineralization of iron sulfides might be controlled by the gastropod to harden the sclerites for protection from predators. Sulfur and iron isotopic analyses indicate that sulfur and iron in the sclerites originate from hydrothermal fluids rather than from bacterial metabolites, and that iron supply is unlikely to be regulated by the gastropod for iron sulfide mineralization. We propose that the gastropod may control iron sulfide mineralization by modulating the internal concentrations of reduced sulfur compounds.
The galatheid crabShinkaia crosnieri, is the sole member of the subfamily Shinkaiinae. It is abundant and forms dense beds around active hydrothermal vents in the Okinawa Trough. Thousands of filamentous bacteria attached to the plumose setae on the ventral surface of this crab were observed using field-emission scanning electron microscopy and transmission electron microscopy. Nucleic acids were extracted from the filamentous bacteria, and the phylotypes of 16S rRNA genes were identified from 81 clones. These phylotypes were divided into three groups: Epsilonproteobacteria (74%); Gammaproteobacteria (20%); and Bacteroidetes (6%). Gamma- and major phylotypes of Epsilonproteobacteria were also detected using fluorescencein situhybridization analysis. These Epsilon- and Gammaproteobacteria were closely related to cultured and uncultured bacteria from hydrothermal vent fields including episymbionts of vent-associated invertebrates such asRimicaris exoculata, Alvinella pompejana, the scaly-foot snail,Kiwa hirsutaetc. The carbon isotopic compositions of the muscle ofS. crosnieriand in filamentous bacteria were similar. The muscle ofS. crosniericontained monounsaturated C16and C18fatty acids, which are known to be characteristic of sulphur-oxidizing bacteria in H2S-rich marine habitats. Through the video images transmitted by a submersible and a remotely operated vehicle,S. crosnieriwas observed to comb out its ventral setae using the third maxilliped and appeared to consume the contents. These evidences suggest the epibiotic association betweenS. crosnieriand the filamentous bacteria attached to the ventral setae of the crab, but the details of role and function are still unclear at the present study.
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