Phylogenetic Characterization of the Endosymbionts of the Deepest-living Vesicomyid Clam, Calyptogena fossajaponica, from the Japan Trench

The bacterial endosymbionts of the vesicomyid clam Calyptogena fossajaponica, collected from the hadal zone in the Japan Trench, were characterized. Transmission electron microscopic (TEM) observations showed numerous bacteria in epithelial cells of the gill tissues of C. fossajaponica. Sulfur content and isotope analyses of this clam also suggested the presence of sulfur-oxidizing bacteria (thioautotrophs). Phylogenetic analysis based on the 16S ribosomal RNA gene (rDNA) sequence indicated that the bacteria was related to sulfur-oxidizing endosymbionts of other vesicomyid clams from deep-sea hydrothermal vents and cold seeps. The symbionts of C. fossajaponica formed a monophyletic group with the symbionts of C. magnifica, C. phaseoliformis and Calyptogena sp. (Florida Escarpment). These 4 species live relatively deeper than other vesicomyid clams whose symbionts have been phylogenetically characterized. Vertical distribution patterns of vesicomyid clams are likely influenced by their symbionts.