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What is a bivalve mollusk?
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What is a bivalve mollusk? Bivalve mollusks (e.g., clams, oysters, mussels, scallops) have an external covering that is a two-part hinged shell that contains a soft-bodied invertebrate. A roughfile clam from the Flower Garden Bank National Marine Sanctuary—just one of many different bivalve mollusk species.
Bivalvia (/ b aɪ ˈ v æ l v i ə /), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bivalves have no head and they lack some usual molluscan organs, like the radula and ...
- Overview
- Size range and diversity of structure
- Distribution and abundance
bivalve, (class Bivalvia), any of more than 15,000 species of clams, oysters, mussels, scallops, and other members of the phylum Mollusca characterized by a shell that is divided from front to back into left and right valves. The valves are connected to one another at a hinge. Primitive bivalves ingest sediment; however, in most species the respira...
Bivalves range in size from about one millimetre (0.04 inch) in length to the giant clam of South Pacific coral reefs, Tridacna gigas, which may be more than 137 centimetres (54 inches) in length and weigh 264 kilograms (582 pounds). Such an animal may have a life span of about 40 years.
The shell morphology and hinge structure are used in classification. In most surface-burrowing species (the hypothetical ancestral habit) the shells are small, spherical or oval, with equal left and right valves. In deeper-burrowing species the shells are laterally compressed, permitting more rapid movement through the sediments. The shells of the most efficient burrowers, the razor clams Ensis and Solen, are laterally compressed, smooth, and elongated. Surface-burrowing species may have an external shell sculpture of radial ribs and concentric lines, with projections that strengthen the shell against predators and damage.
A triangular form, ventral flattening, and secure attachment to firm substrates by byssal threads (byssus; proteinaceous threads secreted by a gland on the foot) have allowed certain bivalves to colonize hard surfaces on wave-swept shores. The byssus is a larval feature that is retained by adults of some bivalve groups, such as the true mussels (family Mytilidae) of marine and estuarine shores and the family Dreissenidae of fresh and estuarine waters. Such a shell form and habit evolved first within sediments (endobyssate), where the byssus serves for anchorage and protection when formed into an enclosing nest. Other bivalves have used the byssus to attach securely within crevices and thus to assume a laterally flattened, circular shape. The best example of this is the windowpane shell Placuna. This form has allowed the close attachment of one valve to a hard surface, and although some groups still retain byssal attachment (family Anomiidae), others have forsaken this for cementation, as in the true oysters (family Ostreidae), where the left valve is cemented to estuarine hard surfaces. Some scallops (family Pectinidae) are also cemented, but others lie on soft sediments in coastal waters and at abyssal depths. By limiting shell thickness (which reduces weight), smoothing the shell contours (which reduces drag), and assuming an aerofoil-like leading edge, such scallops can awkwardly swim several metres at a time.
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In other species, such as the clams, the foot has become modified for rapid and effective digging, and the folds of the mantle tissue have developed into long siphons. Both these features allow the animals to burrow deeply within sand, mud, and other substrates (even into wood and rock). They are protected from predators within such substrata but are still able to feed and breathe using their long siphons.
Most bivalves are marine and occur at all depths in or upon virtually all substrates. In shallow seas, bivalves are often dominant on rocky and sandy coasts and are also important in offshore sediments. They occur at abyssal and hadal depths, either burrowing or surface-dwelling, and are important elements of the midoceanic rift fauna. In addition, bivalves bore into soft shales and compacted muds but may be important also in the bioerosion of corals. Bivalves thus occur at all latitudes and depths, although none are planktonic. There are also estuarine bivalves, and two important families, the Unionidae and Corbiculidae, are predominantly freshwater with complicated reproductive cycles. There are no terrestrial bivalves, although some high-intertidal and freshwater species can withstand drought conditions.
To be expected within a class comprising more than 15,000 living species, abundance varies considerably. Commensal and parasitic species are small, often highly host-specific, and comprise some of the rarest animals. Others, such as cockles and clams on soft shores and mussels and oysters on rocky coasts, can occur in densities high enough that they dominate entire habitats and assume important roles in nutrient cycles.
Phylum Mollusca, Class Bivalvia. Common names of representatives: clams, scallops, oysters, mussels. Habitat (s): marine (salt water), freshwater (lakes, rivers, and streams). Feeding type (s): mostly suspension feeders; some deposit feeders and carnivores. Geological range: Cambrian to today.
bivalve, Any member of the mollusk class Bivalvia, or Pelecypoda, characterized by having a two-halved (valved) shell. Clams, cockles, mussels, oysters, scallops, and shipworms are bivalves. Most are completely enclosed by the shell, the two valves of which are joined by an elastic ligament, and by two sheets of tissue called the mantle.
Jan 26, 2001 · A bivalve is characterized by possessing two shells secreted by a mantle that extends in a sheet on either side of the body. The oldest part of the shell, the umbo, can be recognized as a large hump on the anterior end of the dorsal side of each shell. The two shells are joined at the dorsal end by a region called the ligament.
The second most diverse group of molluscs behind gastropods, bivalves are one of the most important members of most marine and freshwater ecosystems. In fact, there are well over 10,000 described species of bivalve, found from the deepest depths of the oceans, to the streams in your backyard. Bivalves are easily recognized by their two-halved ...
