bivalve
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- What are bivalves?
- What are the common physical characteristics of bivalves?
- Where can bivalves typically be found?
- What is the function of the two shells in bivalves?
- How do bivalves obtain their food?
- What are some examples of bivalves people commonly know?
- How do bivalves reproduce?
- What roles do bivalves play in ecosystems?
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bivalve, (class Bivalvia), any of more than 15,000species ofclams,oysters,mussels,scallops, and other members of the phylumMolluscacharacterized 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 respiratory gills have become modified into organs of filtration called ctenidia. In keeping with a largely sedentary and deposit-feeding or suspension-feeding lifestyle, bivalves have lost the head and the radular rasping organ typical of most mollusks.
General features
Size range and diversity of structure
Bivalves range in size from about one millimetre (0.04 inch) in length to the giantclam 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 ananimal may have a life span of about 40 years.
The shellmorphology and hinge structure are used inclassification. In most surface-burrowing species (thehypothetical 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 clamsEnsis andSolen, 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 threadssecreted 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 (familyMytilidae) of marine and estuarine shores and the family Dreissenidae of fresh and estuarine waters. Such ashell 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 thewindowpane shellPlacuna. This form has allowed the close attachment of one valve to a hard surface, and although some groups still retain byssal attachment (familyAnomiidae), others have forsaken this for cementation, as in the true oysters (familyOstreidae), 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 shellcontours (which reduces drag), and assuming an aerofoil-like leading edge, such scallops can awkwardly swim several metres at a time.
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.
Bivalve shell and body form is thus intimately related to habitat and the relative degree of exposure topredation. From the simple burrowing, equivalve ancestor, the various bivalve groups have repeatedly evolved an elongated, triangular or circular shell; thus, similar bodyadaptations have been responses to similar modes of life.
Distribution and abundance
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, theUnionidae 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 classcomprising more than 15,000 living species, abundance varies considerably. Commensal and parasitic species are small, often highly host-specific, andcomprise some of the rarest animals. Others, such ascockles 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.