Bivalvia, commonly referred to as bivalves, are the class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. They include clams, oysters, mussels, scallops, and numerous other families. The majority are filter feeders and have no head or radula. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed, where they are safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. A few bore into wood, clay or stone and live inside these substances. Some bivalves, such as the scallops, can swim.
The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge by a flexible ligament that, in conjunction with interlocking "teeth" on each of the valves, forms the hinge. This arrangement allows the shell to be opened and closed without the two halves becoming disarticulated. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane. Adult shell sizes vary from fractions of a millimetre to over a metre in length, but the majority of species do not exceed 10 cm (4 in).
Bivalves have long been a part of the diets of coastal human populations. Oysters were cultured in ponds by the Romans, and mariculture has more recently become an important source of bivalves for food. Modern knowledge of molluscan reproductive cycles has led to the development of hatcheries and new culture techniques. A better understanding of the hazards of eating raw and undercooked shellfish has led to improved storage and processing. Besides their use as food, oysters are the most common source of natural pearls. The shells of bivalves are used in craftwork and the manufacture of jewellery and buttons. Bivalves have also been used in the biocontrol of pollution.
Bivalves appear in the fossil record first in the early Cambrian more than 500 million years ago. The total number of living species is approximately 9,200. These species are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water and estuarine species) represent about 8,000 species, combined in four subclasses and 99 families with 1,100 genera. The largest recent marine families are Veneridae, with more than 680 species and the Tellinidae and Lucinidae, each with over 500 species. The freshwater bivalves include seven families, the largest of which is the Unionidae with about 700 species.
Read more about Bivalvia: Etymology, Anatomy, Comparison With Brachiopods, Evolutionary History, Diversity of Extant Bivalves, Distribution, Behaviour, Mariculture, Use As Food, Use in Controlling Pollution, Other Uses, Bivalvian Taxonomies