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.

‘Pliocardia' krylovata, sp. nov. (Bivalvia: Vesicomyidae: Pliocardiinae) is described from cold seeps off the coast of the Nicoya Peninsula, Costa Rica (700–1000 m depth). The phylogenetic position of ‘P.' krylovata was assessed by both morphological comparisons as well as nucleotide data from the cytochrome c oxidase I gene. Within the vesicomyids, ‘P.' krylovata belongs to the Pliocardiinae and its closest relative is ‘Calyptogena' ponderosa, which also bears some morphological resemblance to the genus Pliocardia, perhaps suggesting a need for reanalysis of not only its generic designation, but also the entire Pliocardiinae subfamily. ‘P.' krylovata has morphological similarities to ‘Pliocardia' bowdeniana and ‘Vesicomya' crenulomarginata, recently reassigned to the genus Pliocardia, including a thick shell, obvious rostrum, pointed posterior end, and a sculptured shell with concentric ribs on the outer surface, to name a few. It is morphologically distinguished, however, by having a complex pallial sinus and remarkably deep escutcheon.

A new species of vesicomyid bivalve (Isorropodon nyeggaensis sp. nov.) is described based on shell morphology, from the Nyegga cold methane seep area on the Norwegian continental margin. This is the first description of vesicomyids from the Norwegian Sea and the northernmost record of recent representatives of the family Vesicomyidae. A dispersion of the genus into the Norwegian Sea basin from the north-eastern Atlantic is suggested. A brief description of other macrofauna from methane seep sites at Nyegga is also given.

Calyptogena gallardoi is a vesicomyid bivalve inhabiting a methane seep area located at a depth of 740–870 m off the Bay of Concepción, Chile. Vesicomyids host chemoautotrophic sulphide-oxidizing endosymbiont bacteria and are always found associated to reducing environments. In this study, the gonadal structure and the gametes produced by C. gallardoi are described. Light microscopy is used to examine serial histological sections of the gonads, and scanning electron microscopy is used to visualize the external morphology of gametes. The gonads of both males and females are organized in ramified tubular acini. In males, mature sperm are stored near genital openings in acini lined with a secretor epithelium that resembles a seminal receptacle. Spermatozoids have bullet-like heads with an average length of 30.3±2.6 μm (mean±1 SD). In females, the mature oocytes are driven toward the genital opening through evacuator conduits lined by ‘paddle’ cilia. The average diameter of oogonias is 11.6±2.5 μm and that of mature oocytes is 273.8±23.1 μm, making the size of the mature oocyte among the largest reported for bivalves. In addition, C. gallardoi is shown to have external sexual dimorphism. Shells of males are significantly smaller and more elongated with sloping postero-dorsal margin compared with shells of females. The data are discussed in the context of available information on reproductive biology of vesicomyids.