bivalve

Bivalvia, commonly called bivalves, is a class of aquatic mollusks whose body is encased in a pair of hinged shells. The two shells are usually symmetric and connected by a hinge and strong adductor muscles that close the shells. Most bivalves lack a distinct head and radula, and their bodies are laterally compressed within the shell. The group includes clams, oysters, mussels, and scallops.

Anatomy and feeding: A large mantle cavity houses the gills, which function in respiration and filter feeding. Cilia on the gills trap suspended particles and transport them to the mouth. Water is drawn in through incurrent siphons and expelled through excurrent siphons in many species. A muscular foot is used for anchoring in sediment and for limited movement; the adductor muscles keep the shell closed when needed.

Habitat and lifestyle: Bivalves inhabit marine and freshwater environments, from tidal flats to deep oceans. Many species are sessile as adults, attaching to substrates or living buried in mud or sand, while scallops can swim by rapid clapping of their shells.

Reproduction and development: Most bivalves are dioecious; fertilization is often external. Early development includes free-swimming trochophore and veliger larval stages. Some freshwater groups produce a parasitic larval stage called glochidia that attach to fish gills or fins before metamorphosis.

Ecology and human use: Bivalves are important filter feeders that help maintain water quality in aquatic ecosystems. They are widely consumed by humans and are raised in aquaculture, notably oysters, mussels, and clams. Pearls are produced by some oyster species. They face threats from overharvesting, habitat loss, pollution, and invasive species such as zebra mussels, and several species are protected or managed to support sustainability.