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A bivalve is a type of aquatic mollusk with a two-sided, symmetrical shell. Each side of the shell is a valve; hence gastropods, such as snails, are sometimes called univalves. Some common bivalve species include clams, oysters, and mussels. There are both freshwater and marine bivalve species.
Though bivalves are superficially similar to another group of aquatic animals, brachiopods, they have distinct anatomical differences. In a brachiopod, the two halves of the shell indicate the top and bottom of the animal's body, while in a bivalve, the two sides of the shell are the left and right halves. In addition, bivalves have much more morphological sophistication and diversity than brachiopods. For example, many have evolved to be mobile, whereas all brachiopods are sessile, or attached to the substrate.
The bivalve first appears in the fossil record around 520 million years ago, near the end of a period known as the Cambrian Explosion. Brachiopods appear earlier in the Cambrian period and are much more numerous than bivalve mollusks throughout the Paleozoic era. However, after 96% of brachiopod genera went extinct during the end-Permian extinction event marking the close of the Paleozoic era, bivalves began to dominate the scene. Bivalve species survived the extinction event better than most groups, with only 59% of bivalve genera dying out.
The impressive survival rate of bivalves during the end-Permian extinction event, as well as their later proliferation and current success, are probably related to their many sophisticated adaptations to marine life. Instead of using muscles to open and close their shells, as brachiopods do, bivalves use a muscle to close their shell, but open naturally when the muscle is relaxed due to the help of a ligament. This adaptation allows the bivalve to conserve energy. Bivalves lack a radula, the toothed tongue-like structure that other mollusks use to eat, and instead are filter-feeders.
Many bivalve species are mobile, though they move in a variety of ways. Some propel themselves by opening and closing, sucking in and expelling water. Others have a foot that helps them move and dig. Many bivalve species can burrow into sand, or even rock or wood, allowing them to evade predators. Some of the more mobile varieties become predators themselves.