Bivalves

Bivalvia is a class of marine and freshwater mollusks that have a laterally compressed shell composed of to valves (left and right) joined dorsally by an elastic ligament. This ligament 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 valves becoming disarticulated. Bivalves typically display bilateral symmetry both in shell and anatomy, but there are significant departures from this theme such as scallops and oysters.

This class is the second most diverse group of mollusks behind gastropods. Adult shell sizes vary from one millimeter to over a meter in length, but the majority of species do not exceed 10 cm.

The name "bivalve" is derived from the Latin bis, meaning "two", and valvae, meaning "leaves of a door". Not all animals with shells with two hinged parts are classified under Bivalvia; other animals with paired valves include certain gastropods (small sea snails in the family Juliidae) and some members of the phylum Brachiopoda (image).

California clam shrimp, Cyzicus californicus

Bivalves have long been a part of the diets of coastal human populations. Oysters were cultured by the Romans, and mariculture has more recently become an important source of bivalves for food. Besides their use as food, oysters are the most common source of natural pearls. The shells of bivalves are used in craftwork and the manufacture of jewelery and buttons. Bivalves have also been used in the biocontrol of pollution.

Bivalves appear in the fossil record first in the early Cambrian more than 500 million years ago. The total number of living species is approximately 9,200 that inhabit most aquatics habitats. The marines species range from shallow to the deep waters, from equator to the poles. Bivalves also have colonized brackish-water estuaries and fresh water rivers, streams, and lakes. 

The bivalve shell consists of two valves that are hinged dorsally, usually with interlocking teeth (the hinge), and always with a horny ligament that connects the two valves along their dorsal surfaces and acts to force the valves apart. The interior of the valves contain scars of the various muscles attached to it, in particular the (usually two, sometimes one) adductor muscles that, on contraction, close the valves. The shell can also be internal, reduced or even absent as in shipworms.

Shipworm

Near the hinge of the shell is the umbone or beak, a rounded, knobbly protuberance. This represents the oldest portion of the shell. 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.

The valves of bivalves are made of either calcite, as is the case in oysters, or both calcite and aragonite. The ligament and byssus, if calcified, are composed of aragonite. The shell usually consists of three layers: an outer periostracum, and outer an inner shell layers.The outermost layer of the shell is the periostracum, a skin-like layer which is composed of a hard organic substance and is usually olive or brown in colour and easily abraded. The outer layer forms surface details such as scales or spines.

In all mollusks, 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.

Most bivalves have a capacious mantle cavity that accommodates large gills, but they don't have head, radula or jaws. In addition to respiration, the gills filter food particles from the water in the majority of bivalves. Some primitive forms feed directly on the organic matter in fine sediments, a few specialized groups derive nutrition from symbiotic algae or bacteria, while others capture and consume small crustaceans and worms in the deep sea.

The intestine is irregularly looped and opens dorsally into the exhalant area. Also opening into this region are the paired kidneys and, when separate from the kidneys, the gonopores of the paired gonads. The heart typically lies below the center of the valves and consists of two auricles and a single ventricle that supplies both anterior and posterior aorta. The nervous system is made up of three pairs of ganglia.

The bivalve foot is modified as a powerful digging tool in many groups, while in those that live a permanently attached life (e.g., oysters), it is very reduced.

Most bivalves burrow in sand or mud, some in wood, clay or coral. Some attach to hard substrates with threadlike strands of protein (byssus), others by permanently cementing one of their valves.

The tiny larva of a bivalve produces a single, uncalcified, caplike shell,called a pellicle. As the larva growths, it is gradually enveloped by two mantle lobes, each developing a separate center of calcification – the dissoconchs, the parts of the shell produced after the larva metamorphoses, assume the proportions and features of the adult bivalve.

Bivalves are divided into the following subclasses:

Protobranchia
Pteriomorpha

Anomalodesmata

Rostroconchia
Heterodonta

Palaeoheterodonta

Chama limbula (Heterodonta)

Spondylus regius (Pteriomorphia)