Methane‐based symbiosis in a mussel, Bathymodiolus platifrons, from cold seeps in Sagami Bay, Japan

Abstract. Bathymodiolus platifrons , a chemosynthetic mussel from cold seeps off Japan, relies for its nutrition on the productivity of methylotrophic or methanotrophic endosymbionts. High densities of bacterial symbionts appearing to be type I methanotrophs were observed in transmission electron micrographs of gill tissues. Methanol dehydrogenase activity in gill tissue from a single individual was positive compared to non‐methanotrophic control samples, indicating a high potential for methanotrophy. Stable isotopic ratios of carbon in symbiont‐containing gill tissue, as well as host tissues, were extremely depleted in 13 C, and similar to values reported for other methanotrophic species. TEMs of gill tissue showing symbionts in various stages of digestion support the hypothesis that carbon transfer from symbionts to B. platifrons occurs through intracellular digestion of the symbionts. Discovery of methane‐ or methanolbased symbioses in B. platifrons from cold seeps in Sagami Bay extends the range of such symbioses to include cold seeps and hydrothermal vents, and supports the idea that environmental methane levels control the distribution of these symbioses.