Overview
The Bivalvia is the second largest class of molluscs. Bivalves are bilaterally symmetrical, laterally compressed molluscs, with extensive mantle lobes which secrete a single shell composed of two valves. The two valves of the shell are hinged dorsally where they are held together by a ligament, and usually completely enclose the rest of the body. They are the only molluscs which lack a distinct head.
The bivalves are mainly marine, but a few species are found in freshwater habitats. They occur from high mountain streams to the deepest oceanic trenches, but they are entirely aquatic.
Bivalves (clams, mussels, oysters etc.), are characterised by a pair of shells (left and right valves), and the lack of a head, although the nervous system is well-developed and some species have eyes. (A scallop's row of bead-like eyes can just be seen between the open shell valves.)
Bivalves display a great diversity of shell shape and size, ranging from 1 millimetre in length to the metre long giant clams of tropical coral reefs. Of the thousands of species worldwide (1,314 species in the New Zealand region alone), the majority are only a few millimetres in length. In the New Zealand region, the largest bivalve is the horse mussel, Atrina zelandica, which may reach 450 mm in length. Most are filter feeders, while some are carnivorous.
Bivalves live by attaching to hard surfaces with cement or fibres; burrowing in sediments; boring in rocks, wood or shells; as external or internal commensals or parasites; or free-swimming. Nourishment is by filter feeding, deposit feeding, carnivory, parasitism, or indirectly from plants or methane or sulphur metabolising bacteria growing within their tissues.