Bivalvia, commonly referred to as bivalves, are the class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. They include clams, oysters, mussels, scallops, and numerous other families. The majority are filter feeders and have no head orradula. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed, where they are safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. A few bore into wood, clay or stone and live inside these substances. Some bivalves, such as the scallops, can swim.
The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge by a flexible ligament that, in conjunction with interlocking “teeth” on each of the valves, forms the hinge. This arrangement allows the shell to be opened and closed without the two halves becoming disarticulated. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane. Adult shell sizes vary from fractions of a millimetre to over a metre in length, but the majority of species do not exceed 10 cm (4 in).
Bivalves vary greatly in overall shape. Some, such as the cockles, are nearly globular and can jump by bending and straightening the foot. Others, such as the razor clams, are burrowing specialists with elongated shells and powerful feet for digging. The shipworms of the family Teredinidae have greatly elongated bodies, but the shell valves are much reduced and restricted to the anterior end of the body, where they function as scraping organs that permit the animal to dig tunnels through wood.
Near the hinge of the shell is the umbone or beak, a rounded, knobbly protuberance. This represents the oldest portion of the shell, with extra material later being laid down along the margins on the opposite side. The hinge area is the dorsal region of the shell and the lower margin is the ventral region. The anterior or front of the shell is where the byssus and foot are located, and the posterior of the shell is where the siphons are located. When the umbone is uppermost, the valve with the anterior end to the left is considered to be the left valve, while the valve with the anterior end to the right is the right valve.
In all molluscs, the mantle forms a thin membrane covering the animal’s body and extending out from it in flaps or lobes. In bivalves, the mantle lobes secrete the valves, and the mantle crest secretes the whole hinge mechanism consisting of ligament, byssus threads, and teeth.
Visible on the inside of most empty bivalve valves is a shiny line that runs parallel to the outer margin of the shell and often connects the two adductor muscle scars. This line (known as the pallial line) exists because parallel to the opening edge of the bivalve’s shell, the mantle is attached to the shell by a continuous narrow row of minute mantle retractor muscles. The function of these small muscles is to pull the loose edge of the mantle up out of harm’s way when this is necessary because of minor predation attempts. In many bivalves, the mantle edges fuse at the posterior end of the shell to form two siphons, through which water is inhaled and expelled for respiration and suspension feeding. There is a pocket-like space into which the siphons fit when they are retracted. This is visible on the inside of the valve as an indentation on the pallial line which is known as the pallial sinus.
The shell is composed of two calcareous valves held together by a ligament. The valves are made of either calcite, as is the case in oysters, or both calcite and aragonite. Sometimes the aragonite forms an inner, nacreous layer, as is the case in the order Pterioida. In other taxa, alternate layers of calcite and aragonite are laid down. The ligament and byssus, if calcified, are composed of aragonite. The outermost layer of the shell is theperiostracum, a skin-like layer which is composed of a horny organic substance. The periostracum is secreted in the groove between the outer and middle layers of the mantle, and is usually olive or brown in colour and easily abraded. The outer surface of the valves is often sculpted with clams having fine concentric striations, scallops radial ribs and oysters a latticework of irregular markings.
The shell is added to in two ways; the valves grow larger when more material is secreted by the mantle at the margin of the shell, and the valves themselves thicken gradually throughout the animal’s life as more calcareous matter is secreted by the mantle lobes. The two valves are held together at a hinge joint by a ligament composed of two keratinised proteins, tensilium and resilium. In different groups of bivalves the ligament can be internal or external in position. The main function of the ligament (as well as joining the valves together) is to passively cause the shell to open. The shell is actively closed using the adductor muscle or muscles attached to the inner surface of both valves. The position of the muscles is often clearly visible on the inside of empty valves as circular or oval muscle scars. Along the hinge line of the shell there are often a number of hinge teeth which prevent the valves from moving laterally relative to one another. The arrangement of these teeth is often important in identifying bivalves.
The sensory organs of bivalves are not well developed and are largely located on the posterior mantle margins. The organs are usually mechanoreceptors or chemoreceptors located in shorttentacles. The chemoreceptor cells taste the water and are sensitive to touch. They are typically found near the siphons, but in some species may fringe the entire mantle cavity. Theosphradium is a patch of sensory cells located below the posterior adductor muscle that may serve to taste the water or measure its turbidity, but is probably not homologous with the structure of the same name found in snails and slugs. Statocysts within the organism help the bivalve to sense and correct its orientation. Each statocyst consists of a small sac lined with sensory cilia that detects the movement of a mineral mass, a statolith, under gravity. In the order Anomalodesmata, the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey.
Many bivalves have no eyes, but a few members of Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea and Limoidea have simple eyes on the margin of the mantle. These consist of a pit of photo-sensory cells and a lens. Scallops have more complex eyes with a lens, a two-layered retina and a concave mirror. All bivalves have light-sensitive cells that can detect a shadow falling over the animal.
The main muscular system in bivalves is the posterior and anterior adductor muscles, although the anterior muscles may be reduced or even lost in some species. These strong muscles connect the two valves and contract in order to close the shell. They work in opposition to the ligament which tends to pull the valves apart. In sedentary or recumbent bivalves that lie on one valve, such as the oysters and scallops, the anterior adductor muscle has been lost and the posterior muscle is positioned centrally. In file shells that can swim by flapping their valves, there is also a single, central adductor muscle. These muscles are composed of two types of muscle fibre, striated muscle bundles for fast actions and smooth muscle bundles for maintaining a steady pull.
The mantle suspender muscles attach the mantle to the shell and leave an arc-shaped scar on the inside of the valve, the pallial line. The paired pedal protractor and retractor muscles operate the animal’s foot. Some bivalves, such as oysters and most scallops, are unable to extend their foot and in them, these muscles are absent. Other paired muscles control the siphons and the byssus.
Most bivalves are filter feeders, using their gills to capture particulate food such as phytoplankton from the water. The Protobranchs feed in a different way, scraping detritus from the seabed, and this may be the original mode of feeding used by all bivalves before the gills became adapted for filter feeding. These primitive bivalves hold onto the substratum with a pair of tentacles at the edge of the mouth, each of which has a single palp, or flap. The tentacles are covered in mucus, which traps the food, and cilia, which transport the particles back to the palps. These then sort the particles, rejecting those that are unsuitable or too large to digest, and conveying others to the mouth.
In the Filibranchia and Eulamellibranchia, water is drawn into the shell from the posterior ventral surface of the animal, passes upwards through the gills and doubles back to be expelled just above the intake. In burrowing species, there may be two elongated, retractable siphons reaching up to the seabed, one each for the inhalant and exhalant streams of water. The gills of filter-feeding bivalves are known as ctenidia and have become highly modified to increase their ability to capture food. For example, the cilia on the gills, which originally served to remove unwanted sediment, have become adapted to capture food particles, and transport them in a steady stream of mucus to the mouth. The filaments of the gills are also much longer than those in more primitive bivalves, and are folded over to create a groove through which food can be transported. The structure of the gills varies considerably, and can serve as a useful means for classifying bivalves into groups.
A few bivalves, such as the granular poromya (Poromya granulata), are carnivorous, eating much larger prey than the tiny microalgae consumed by other bivalves. In these animals, the gills are relatively small, and form a perforated barrier separating the main mantle cavity from a smaller chamber through which the water is exhaled. Muscles draw water in through the inhalant siphon which is modified into a cowl-shaped organ, sucking in small crustaceans and worms at the same time. The siphon can be retracted quickly and inverted, bringing the prey within reach of the mouth. The gut is modified so that large food particles can be digested.
The sexes are usually separate in bivalves but some hermaphroditism is known. The gonads are located close to the intestines, and either open into the nephridia, or through a separate pore into the mantle cavity. The ripe gonads of male and females release sperm and eggs into the water column. Spawning may take place continually or be triggered by environmental factors such as day length, water temperature or the presence of sperm in the water. Some species are “dribble spawners” but others release their gametes in batches or all at once. Mass spawning events sometimes take place when all the bivalves in an area synchronise their release of spawn.
Fertilization is usually external. Typically, there is a short stage lasting a few hours or days before the eggs hatch into trochophore larvae. These later develop into veliger larvae which settle on the seabed and undergo metamorphosis into juveniles known as spat. In some species, such as those in the genus Lasaea, females draw water containing sperm in through their inhalant siphons and fertilisation is inside the female. These species then brood the young inside their mantle cavity, eventually releasing them into the water column as veliger larvae or as crawl-away juveniles.
The bivalves are a highly successful class of invertebrates found in aquatic habitats throughout the world. Most are infaunal and live buried in sediment on the seabed, or in the sediment in freshwater habitats. A large number of bivalve species are found in the intertidal and sublittoral zones of the oceans. A sandy sea beach may superficially appear to be devoid of life, but there is often a very large number of bivalves and other invertebrates living beneath the surface of the sand. On a large beach in South Wales, careful sampling produced an estimate of 1.44 million cockles (Cerastoderma edule) per acre of beach.
Bivalves inhabit the tropics as well as temperate and boreal waters. A number of species can survive and even flourish in extreme conditions. They are abundant in the Arctic, about 140 species being known from that zone. The Antarctic scallop, Adamussium colbecki, lives under the sea ice at the other end of the globe, where the sub-zero temperatures mean that growth rates are very slow. The giant mussel, Bathymodiolus thermophilus, and the giant white clam, Calyptogena magnifica, both live clustered around hydrothermal vents at abyssal depths in the Pacific Ocean. They havechemosymbiotic bacteria in their gills that oxidise hydrogen sulphide, and the molluscs absorb nutrients synthesized by these bacteria. The saddle oyster, Enigmonia aenigmatica, is a marine species that could be considered amphibious. It lives above the high tide mark in the tropical Indo-Pacific on the underside of mangrove leaves, on mangrove branches and on sea walls in the splash zone.
Most bivalves adopt a sedentary or even sessile life style, often spending their whole lives in the area in which they first settled as juveniles. The majority of bivalves are infaunal, living under the seabed, buried in soft substrates such as sand, silt, mud, gravel or coral fragments. Many of these live in the intertidal zone where the sediment remains damp even when the tide is out. When buried in the sediment, burrowing bivalves are protected from the pounding of waves, desiccation and overheating during low tide, and variations in salinity caused by rainwater. They are also out of the reach of many predators. Their general strategy is to extend their siphons to the surface for feeding and respiration during high tide, but to descend to greater depths or keep their shell tightly shut when the tide goes out. They use their muscular foot to dig into the substrate. Other bivalves, such as mussels, attach themselves to hard surfaces using tough byssus threads made of keratin and proteins. They are more exposed to attack by predators than the burrowing bivalves. Some bivalves, including the true oysters, the jewel boxes, the jingle shells, the thorny oysters and the kitten’s paws, cement themselves to stones, rock or larger dead shells. In oysters the lower valve may be almost flat while the upper valve develops layer upon layer of thin horny material reinforced with calcium carbonate. Oysters sometimes occur in dense beds in the neritic zoneand, like most bivalves, are filter feeders.
Razor shells can dig themselves into the sand with great speed to escape predation. When a Pacific razor clam (Siliqua patula) is laid on the surface of the beach it can bury itself completely in seven seconds  and the Atlantic jackknife clam, Ensis directus, can do the same within fifteen seconds. Scallops and file clams can swim by opening and closing their valves rapidly; water is ejected on either side of the hinge area and they move with the flapping valves in front. Scallops have simple eyes around the margin of the mantle and can clap their valves shut to move sharply, hinge first, to escape from danger. Cockles can use their foot to move across the seabed or leap away from threats. The foot is first extended before being contracted suddenly when it acts like a spring, projecting the animal forwards.
In many bivalves that have siphons, they can be retracted back into the safety of the shell. If the siphons inadvertently get attacked by a predator, they snap off. The animal can regenerate them later, a process that starts when the cells close to the damaged site become activated and remodel the tissue back to its pre-existing form and size.
File shells such as Limaria fragilis can produce a noxious secretion when stressed. It has numerous tentacles which fringe its mantle and protrude some distance from the shell when it is feeding. If attacked, it sheds tentacles in a process known as autotomy. The toxin released by this is distasteful and the detached tentacles continue to writhe which may also serve to distract potential predators.
Notes from Wikipedia. For more details: http://en.wikipedia.org/wiki/Bivalvia