Heterodonta

Heterodonta is a traditional taxonomic grouping within the bivalve molluscs (Bivalvia). The name derives from the heterodont hinge teeth found in many of its members, distinguishing them from other bivalves. In classical classifications, Heterodonta comprises a diverse array of species, including many clams, cockles, razor shells, and freshwater mussels, inhabiting marine, estuarine, and freshwater environments worldwide.

Morphology and biology: Members typically have two-part shells with a strong adductor muscle and a hinge line often bearing one or more hinge teeth (the heterodont condition). The shells are usually more or less equivalve and laterally compressed. Most species are filter feeders, using ctenidial gills to capture food particles from the water. Many have a well-developed siphon system that allows feeding and respiration while buried in sediment. Reproduction tends to be sexual with various larval stages; some species release free-swimming trochophore and veliger stages, while others have direct development.

Distribution and ecology: The group is globally distributed, from shallow coastal waters to deep seas, as well as freshwater rivers and lakes. They occupy mud, sand, and gravel bottoms, oyster beds, and other substrates. Ecologically important members include filter-feeding mass stabilizers in sediment and, in some groups, symbiotic relationships with bacteria (notably lucinids).

Taxonomy and phylogeny: Heterodonta is a historically recognized subgroup of Bivalvia, though modern molecular studies have revised several relationships and reclassified some lineages. The term remains in use in many field guides and older literature to describe a broad assemblage of bivalves with heterodont hinge dentition.