Dual symbiosis in the cold-seep thyasirid clamMaorithyas hadalisfrom the hadal zone in the Japan Trench, western Pacific

The bacterial endosymbionts of the thyasirid clamMaorithyas hadaliscollected 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 andin situhybridization. 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 (generaThiomicrospiraandHydrogenovibrio).In situhybridization experiments indicated spatial partitioning between the 2M. hadalissymbionts, 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.