Dual symbiosis in the cold-seep thyasirid clam Maorithyas hadalis from the hadal zone in the Japan Trench, western Pacific

The bacterial endosymbionts of the thyasirid clam Maorithyas hadalis collected from the hadal zone in the Japan Trench were characterized. Two distinct phylotypes of endosymbiotic bacteria were discovered within the gill tissues by molecular phylogenetic analysis and in situ hybridization. Symbiont Type I was affiliated with thioautotrophic symbionts of vesicomyid clams and deep-sea mussels from deep-sea hydrothermal vents and cold seeps. Symbiont Type II was not related to previously reported bacterial symbionts, and was distantly related to the free-living chemoautotrophic bacteria (genera Thiomicrospira and Hydrogenovibrio). In situ hybridization experiments indicated spatial partitioning between the 2 M. hadalis symbionts, with Symbiont I occurring mainly in the outer regions of bacteriocyte zones and Symbiont II situated predominantly within inner regions of bacteriocyte zones. This is the deepest chemosynthetic symbiosis ever recorded and the first report to show spatial partitioning between the intracellular endosymbionts in marine invertebrates.