Bivalves (Bivalvia) are a group of molluscs that include clams, scallops, oysters, mussels, razor shells, cockles, venus shells, borers, trough shells and many others (some of which have yet to be identified). In total, there are about 9,200 living species of bivalves making them the second most diverse group of molluscs, ranking behind only the gastropods in species numbers.
Bivalves are so named for their paired shells. The shells of a bivalve are made up of two halves that are mirror images of each other and are joined at one edge by a flexible hinge. Each half is asymmetrical and rounded, so that when closed against the other half, forms a domed space near the hinged edge of the shell which accommodates the bulk of the animal's body and narrows towards the edge of the shell that opens.
Although most bivalves have a shell that consists of two halves, a few either have a reduce shell or no shell at all.
Bivalves inhabit marine and freshwater habitats (most diverse are marine species, with more than 80 percent of the species living in ocean habitats).
There are four different life strategies that bivalves exhibit: epifaunal, infaunal, boring and free-moving. Epifaunal bivalves attach themselves to hard surfaces and remain in the same spot for their entire life. Epifaunal bivalves such as oysters adhere to surfaces using either cementation or byssal threads (sticky chitinous threads secreted by a gland in the foot). Infaunal bivalves such as bury themselves in sand or sediment on the seafloor or in riverbeds. Some species bury themselves just below the surface while others bury themselves deep within the sediment. They may have spines that help to secure them in the sediment and to prevent predators from dislodging them. Boring bivalves have thin, soft shells that are armed with a hard tip. They bore into solid surfaces such as wood or rock. Free-moving bivalves such as scallops use their muscular foot to dig into sand and soft sediments. They can also move through the water by opening and closing their valves, an action that pushes them through the water.
Most bivalves have a pair of large gills which are located in their mantle cavity. The gills enable them to extract oxygen from the water (to breathe) and to capture food. Water is drawn into the mantle cavity and washes through the gills. In species that burrow, a long siphon is extended to the surface to take in water. Mucus on the gills helps capture food and cilia transfer the food particles to the mouth. Once in the mouth, food passes into the stomach to be digested.
Bivalves have a mouth, heart, intestine, gills, stomach and siphon but have no head, radula or jaws. They have an abductor muscle that, when contracted, holds the two halves of the shell closed. Bivalves have a muscular foot, which in many species such as clams, is used to anchor their body to the substrate or dig down into the sand.
There are six groups of bivalves which are referred to here by their scientific name (since they have so many members and, in some cases, common names are not available). These include the Protobranchia, Pteriomorpha, Anomalodesmata, Rostroconchia, Heterodonta and Palaeoheterodonta. The most familiar of these groups is the Pteriomorpha, a group that includes animals such as scallops, oysters, pearl oysters, mussels, arcs and various other families.
The earliest fossils of bivalves date back to the Early Cambrian. During the Early Ordovician, bivalves diversified in terms of both number of species and the variety of ecological niches occupied.