Ultrastructure and stable carbon isotope composition of the hydrothermal vent mussels Bathymodiolus brevior and B. sp. affinis brevior from the North Fiji Basin, western Pacific

In hydrothermal vents of the western Pacific several mytilid bivalve species of the genus Bathymodiolus attain a dominating ecological role. The ultrastructure of the symbiosis in the vent mytilid B. brevior as well as in an undescribed species B. sp, affinis brevior from a deep-sea hydrothermal vent site in the North Fiji Basin of the southwestern Pacific was studied. The abundance of symbionts was strikingly different in the 2 species, with B. brevior gills housing low numbers of bacteria that occurred singly in membrane-bound vacuoles, while B, sp. aff. brevior gills contained numerous symbionts with up to 12 bacteria per vacuole. Bacterial vacuoles at the outermost region of the bacteriocytes occasionally (B. brevior) or regularly (B. sp, aff, brevior) showed an open connection to the exterior, ambient seawater. In both species only a single symbiont morphotype without intracytoplasmatic membranes occurred, suggesting that these mytilids harbor only chemoautotrophic, sulfur-oxidizing bacteria. Stable carbon isotope ratios provided further information on the trophic interactions between the endosymbionts and their hosts. The delta(13)C values of B. brevior gill tissues ranged from -30.8 to -35.8% and were significantly more negative than in non-symbiotic tissues, indicating that the symbionts may not account for the sole source of dietary carbon in these associations.