Revision of the generic placement of two hadal bivalves (Bivalvia: Thyasiridae) from the Japan Trench, with the introduction of a new genus, Tartarothyas

The hadal bivalves from the Japan Trench originally described as Maorithyas hadalis and Parathyasira kaireiae are reassigned to the genera Tartarothyas n. gen. and Spinaxinus, respectively. The shell, anatomy, and fine structure of the ctenidia are described, and based on these the new genus Tartarothyas is introduced. Both species have modified ctenidia to host chemosymbiotic bacteria; those of Tartarothyas n. gen. is partly tubular in structure, whereas those of Spinaxinus is of fleshy lamellae.