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Various invertebrates inhabiting hydrothermal vents harbor thiotrophic endosymbionts that provide the host with nutrients and are possibly involved in the detoxification of harmful sulfides. In this study, we first determined the partial 16S rRNA gene sequence of the thiotrophic symbiont of the deep-sea mussel Bathymodiolus septemdierum, a dominant species at hydrothermal vents in the Izu-Ogasawara (Bonin) area. We then designed a new probe, Bsob692, for fluorescent in situ hybridization (FISH) using regions completely conserved among thiotrophic symbionts of all bathymodiolin mussels and established the protocol for FISH to compare the distribution and amount of the symbiont using an image analysis program that is commercially available. We compared fluorescent intensity in the gill of the mussels collected at different sites and found a higher intensity in specimens collected from a site with higher sulfide concentration. We also compared mussels reared in the presence and absence of sulfide and found that the former had a higher fluorescent intensity.

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Nucleotide sequences of part (1,101 bp) of the mitochondrial cytochrome oxidase c subunit I (COI) gene were determined for two specimens of Calyptogena kawamurai collected in Kashima Nada and Suruga Bay, respectively. These sequences were identical to each other and to those from many individuals of Calyptogena solidissima, i.e., 11 of 12 specimens from a seep area in Nankai Trough, two of 20 from hydrothermal-vent fields in Okinawa Trough, and one of 14 from a seep area on Kuroshima Knoll. The nucleotide sequences of the 5' part (about 700 bp) of the first internal transcribed spacer (ITS-1) also showed a close relationship between C. kawamurai and C. solidissima. The radiating threads on the shell surface that were emphasized in describing C. solidissima are not consistent throughout these local populations. Variation in cardinal dentition was confirmed to be intraspecific by observations of a series of specimens. The shell length-height and shell length-width relationships of both species all fit a single regression line. These results suggest that C. solidissima is a junior synonym of C. kawamurai. The populations of Nankai Trough, Okinawa Trough, and Kuroshima Knoll were shown to be diverging genetically from each other. Populations of Okinawa Trough and Kuroshima Knoll are suggested to have derived independently from the most common haplotype of Nankai Trough.