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

Trape::ium sublael'igalum 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. THE EPIFAUNAL BIVALVE colonizers of the rocky intertidal and sublittoral are variously adapted to their mode of life. Many are permanently byssally attached, whereas others are only temporarily attached as larvae prior to the adoption of, for example, a cemented life-style. On exposed shores many genera are either heteromyarian in form, e.g., Mytilus and Septifer (Mytilacea) (White 1937, Yonge and Campbell 1968); laterally flattened, e.g., Anomia (Anomiacea) (Yonge 1977); or cemented, e.g., Chama (Chamidae) (Yonge 1967) and Cleidothaerus (Anomalodesmata) (Morton 1974). On less exposed, often estuarine boulderor cobble-strewn beaches, bivalves with similar habits are found, e.g., the heteromyarian Braehidontes and X enostrobus (Mytilacea) (Wilson 1967) and the cemented oysters Ostrea, Crassostrea, and Saeeostrea (Ostreacea) (Yonge 1953). Similarly in the sublittoral, the byssally attached heteromyarian bivalves Cardita and Beguina I Manuscript accepted 6 November 1978. 2 University of Hong Kong, Department of Zoology, Pokfulam Road, Hong Kong. 177 (Carditacea) (Yonge 1969) and the cemented Spondylus (Spondylidae), Plieatula (Plicatulidae) (Yonge 1973), and Dimya (Dimyidae) (Yonge 1975) occur. Aside from the bivalves associated commensally with other organisms, such as the colonizers of compound ascidians, e.g., Ryenella (Mytilacea) and Mytilimeria (Anomalodesmata) (Yonge 1952), and the borers of soft rocks and shales, e.g., Barnea and Pholas (Pholadidae) (Purchon 1955b) and Petrieola (Petricolidae) (Purchon 1955a), a third major category of epifaunal bivalves are the nestlers. Nestlers are usually small, secretive, byssally attached bivalves, which are hidden in crevices, in the holdfasts of algae or among masses of larger bivalves. Representatives from a wide range offamilies fall into this category, such as members of the Arcacea (Barbatia, Area), Mytilacea (Modiolus), and Saxicavacea (Hiatella) (Yonge 1971), together with delegates from a range of families belonging to the vast, cosmopolitan, and highly successful order Veneroida. These include members of the Leptonacea (Lasaea) (Oldfield 1955), Gaimardiacea (Neogaimardia) (Morton 1979),