bivalved

Bivalved is an adjective used to describe organisms or structures that are composed of two valves or shells connected by a hinge. In biology, the term most often refers to two-valved shells, but it can apply to any object that has two separable halves.

In a common usage, bivalved refers to the molluscan class Bivalvia, which includes clams, oysters, mussels, and scallops. These animals have a body largely housed between two hinged shells that are typically oriented to the sides rather than top and bottom. The shells are secreted by the mantle and grow along growth lines. Bivalves lack a radula and true head; they are mostly filter feeders, drawing water in and out through siphons or openings while extracting oxygen and food particles from the water. Most species are sedentary or slow-moving as adults, though some, like scallops, can swim by rapid valve advertisement. Reproduction is usually sexual, often with external fertilization, and many have free-swimming larval stages.

The term bivalved is also used outside mollusks to describe a two-part carapace or shell in other groups, such as ostracods (crustaceans) and certain extinct arthropods or fossil shells. In paleontology and comparative anatomy, describing a structure as bivalved emphasizes the two-part, hinged nature without implying close evolutionary relatedness among all such forms.

Etymology: from Latin bi- “two” and valva “a folding door” or “valve.”