The morphology of deep-sea Thyasiridae (Mollusca: Bivalvia) from the Atlantic Ocean

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Abstract
Twenty-five species and three subspecies of the family Thyasiridae (Bivalvia: Lucinacea) are described from the deep Atlantic. They belong to two genera and five subgenera. Eleven of the species and all of the subspecies are described for the first time. The number of demibranchs in the gill and shape of the lobes of the lateral body pouches are characteristic features which are additionally used to clarify taxonomic divisions that have been previously based on shell features alone. One species, Axinus grandis Verrill & Bush (1898), is thought to be the sole living representative of the predominantly Tertiary fossil genus Axinus Sowerby (1821). This genus has been regarded by most previous authors as a synonym of Thyasira Leach (1818). Axinus grandis, as here defined, is morphologically distinct from all other thyasirids and possesses primitive characters. It shows affinities to the lucinacean family Ungulinidae, suggesting that the Thyasiridae may have an origin close to the ungulinid stem. The morphology of the species described here is extremely conservative, all sharing a number of key features. The most important of these is the form of the lateral body pouches. Shell shape and morphology relate in part to different life habits. Some species which have vertically elongate shape are probably immobile deep-burrowers, while others, principally the smallest species, which are horizontally elongate are adapted for a more active existence. All are infaunal. The deep-water species are thought to be pre-adapted to life at depth through their ability to inhabit impoverished deoxygenated habitats. Few morphological differences could be detected between the populations of species that occur in both shallow and deep waters. The small size of all but a few species may be one of their greatest adaptive features. The greatest reduction in size and greatest simplification of morphology are seen in species of the subgenera Thyasira and Parathyasira. The subgenera Axinulus and Mendicula show the greatest radiation in deep water. The latter subgenera are thought to have arisen from the larger subgenera by neoteny. The thyasirids, unlike the deep-sea protobranch and septibranch bivalves, are predominantly inhabitants of slope depths. Very few are truly abyssal. Many have very wide depth distributions extending from shelf to abyssal depths. This, together with the production of planktotrophic larvae, has ensured that most thyasirid species, unlike other deep-sea bivalves, are very widely distributed. Thus species extend into Arctic waters, including the Norwegian Basin and into the Pacific and are truly cosmopolitan.

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