The functional and adaptive morphology of the deep-sea species of the Arcacea (Mollusca: Bivalvia) from the Atlantic

Abstract

Species of two genera,BathyarcaKobelt (1891) andBentharcaVerrill and Bush (1898), occur beyond the shelf-slope break of the continental margins of the Atlantic. Little is known of these species and descriptions of the morphology ofBathyarca glacialis, B. pectunculoides, B. inaequisculpta, B. corpulenta, Bentharca nodulosaandB. asperulaare given. The species ofBathyarcashow a progression in form from an epibyssate nestling habit to an infaunal endobyssate habit. Correlated with this are various morphological changes. Thus the byssus, which is short and stout and controlled by strong retractor muscles in the epibyssateB. glacialis, is long and slender with weak associated musculature in endobyssateB. inaequisculpta. The mantle/shell form is increasingly heteromyarian in character in deeper-living species, with the increasing development of mantle flaps and muscles of the gill axis. In contrast, the species ofBentharcahave largely retained their epibyssate habit. Their morphology is conservative and differs little from shallow-water arcids. Features are found in both genera that are related to the small quantity of suspended food present in the deep sea. These include the lack of glands in mantle tissue, the reduced palps and lack of sorting areas in the stomach. InBathyarca corpulentaalone the (body biomass): (shell volume) ratio is reduced.