These headless molluscs built shells and human economies
Bivalves—clams, oysters, mussels, scallops—enclose soft bodies in hinged calcium-carbonate valves, filtering food from water without a head or radula. About 9,200 living species span half a billion years of fossil history. Romans cultured oysters; modern hatcheries and pearl farming still depend on understanding their reproductive cycles.
Class Bivalvia includes marine and freshwater molluscs with laterally compressed bodies inside paired shells joined by a hinge ligament. Gills evolved into ctenidia that both breathe and filter phytoplankton. Most species burrow in sediment; others attach to rock, swim by flapping valves, or bore into wood and stone as shipworms. Shells of calcite or aragonite grow by secretion from the mantle edge; concentric rings help age individuals.
Anatomy reflects sedentary life. Posterior adductor muscles clamp valves shut against the hinge ligament's opening force. The nervous system lacks a brain, using paired ganglia instead. Many have simple mantle-edge eyes; scallops possess complex eyes with lenses, layered retinas, and concave mirrors. An open circulatory system bathes organs in hemolymph pumped through a three-chambered heart. Siphons inhale and exhale water; the osphradium may taste incoming flow.
Humans have long harvested bivalves for food. Oyster ponds date to Roman times; pearl oysters supply natural pearls; shells become buttons, jewelry, and craft material. Some species serve in pollution biocontrol. Fossils appear in the early Cambrian over 500 million years ago. Marine families alone include roughly 8,000 species—the Veneridae exceeding 680, Tellinidae and Lucinidae each over 500. Linnaeus coined Bivalvia in 1758 for animals with two valves; the older name Pelecypoda meant axe-foot. The everyday word comes from Latin bis and valvae, two leaves of a door, and double shells have arisen separately in brachiopods, tiny ostracod crustaceans, and a family of small sea snails. The roughly 9,200 living species are sorted into 1,260 genera and 106 families, and shipworms shrink their valves to scrapers at the front of a long body so they can tunnel through timber.
Source: Bivalvia
