New species of chemosymbiotic clams (Bivalvia: Vesicomyidae and Thyasiridae) from a putative ‘seep’ in the Hatton–Rockall Basin, north-east Atlantic

Two new species of chemosymbiotic bivalves are described from a poorly localized site in the Hatton–Rockall Basin at 1187–1200 m. The new species Isorropodon mackayi sp. nov. is compared with others of the genus from the North and South Atlantic. A novel anatomical structure, suggested to be a secondary gill, is described. The second bivalve Thyasira scotiae sp. nov. is compared with other species known to inhabit cold seeps in the Atlantic and most resembles Thyasira sarsi. Other than the minor morphological differences, the bathyal range and presence of commensal polychaetes, Antonbrunnia, never reported in T. sarsi support the erection of a new species. In the North Atlantic Isorropodon species and the larger thyasirids, with the exception of T. sarsi, are typically found at hydrocarbon seeps associated with pockmarks and mud volcanoes. If correct, then this is the first indication of active sulphidic seepage in the Hatton–Rockall Basin.