The adaptations and radiation of deep-sea bivalves

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Abstract
Bivalve molluscs represent approximately 10 % of the specimens taken in samples from the abyssal Atlantic. In soft oozes Protobranchia, Septibranchia, and Thya-siridae account for more than 95 % of the total number of bivalves present. Where there is hard bottom, byssally attached forms predominate, particularly species of Bathyarca, Limopsis, and Dacrydium. The Protobranchia dominate. At the shelf/slope break they account for 30 % of the bivalve specimens present, at the slope bottom this increases to 50 % and then to between 80 % and 90 % on the abyssal plain. In numbers of species Protobranchia rarely exceed 60 % of those present, but as many as 14 species may be present at a station. Species are zoned vertically and approximately 60 % are common to both sides of the mid-Atlantic Ridge. The bivalves rarely exceed 5 mm total length. Growth rate is slow and fecundity low. The smallest species are miniaturized. Species from abyssal depths have smaller gills than related species from shelf sediments. Associated with this is an increase in the size of the palps. Other morphological changes in deposit-feeding species involve simplification of the stomach and an increase in hind gut length.