Some Aspects of the Biology and Functional Morphology of Trapezium (Neotrapezium) sublaevigatum (Lamarck) (Bivalvia: Arcticacea)

Abstract

Trapezium sublaevigatum is a widely distributed nestler in rocky intertidal and sublittoral crevices in Hong Kong. It occurs on a range of beach types and in waters of different salinity; it is byssally attached and weakly heteromyarian in form. The quadrangular shell is often distorted. The basic structure of the shell and the morphology of the organs of the mantle cavity, visceral mass, and pericardium are broadly similar to a wide range of veneroid relatives. However, the species is weakly heteromyarian, which results from the assumption of the byssally attached, epifaunal mode of life. There is thus a reduction of the anterior face of the shell and an enlargement of the posterior face with corresponding modifications to the shell (notably hinge teeth and ligament) and to musculature. Similar, convergent adaptations are seen in members of the veneroid superfamilies Carditacea, Gaimardiacea, and Dreissenacea, with which Trapezium can be compared. Further study of other members of the (isomyarian) Arcticacea might help to establish how the heteromyarian form arose in Trapezium.