Symbiotic Adaptation for Deeper Habitats in Chemosynthetic Environments

Abstract

The bacterial endosymbionts of the solemyid clam Acharax johnsoni and the thyasirid clam Parathyasira kaireiae, collected from the Japan Trench, were characterized. Transmission electron microscopic observations showed numerous bacteria in the epithelial cells of the gill tissues of A. johnsoni. Numerous bacteria were also visible in the gill tissues of P. kaireiae, but were not located within the epithelial cells. Phylogenetic analysis based on the 16S ribosomal RNA gene sequences from the gill tissues of both clams indicated that the bacteria were related to sulfur-oxidizing endosymbionts from deep-sea chemosynthetic environments. The symbiont of A. johnsoni formed a monophyletic group with the thioautotrophic symbiont of Solemya reidi, which lives relatively deeper than other solemyid clams. The symbiont of P. kaireiae formed a monophyletic group with symbiont II in Maorithyas hadalis, which lives in the hadal zone of the Japan Trench. In addition, four vesicomyid species living relatively deeper than other vesicomyid clams also have a specific clade of thioautotrophic symbionts. Bacterial chemotrophic endosymbionts as well as the mode of symbiosis might influence host distributions in deep-sea chemosynthetic ecosystems.