Bivalve shell

The bivalve shell is one kind of seashell, and in life it is composed of two parts, two valves. The bivalve shell is part of the body, the exoskeleton, of a bivalve mollusk.

Bivalves are a common part of the marine fauna worldwide (scallops, clams, oysters, mussels, etc) and are also quite common in freshwater. The shells of bivalves wash up on beaches (often as separate valves) and are also found along the flood plains of rivers, and other freshwater habitats.

Bivalves typically have two-part shells, two valves, that are joined by a ligament. The two valves usually articulate with one another using structures known as "teeth" which are situated along the hinge line. In many (but by no means all) bivalve shells, the two valves are symmetrical along the hinge line.

This exoskeleton serves not only for muscle attachment, but also for protection from predators and from mechanical damage. The shell has several layers, and is typically made of calcium carbonate precipitated out into an organic matrix. It is secreted by a part of the molluscan body known as the mantle.

Bivalve shells are collected by professional and amateur conchologists, and are sometimes harvested for commercial sale (the international shell trade), occasionally to the detriment of the local ecology.

Shell anatomy, structure and composition

The bivalve shell is composed of two calcareous valves. The mantle, a thin membrane surrounding the body, secretes the shell valves, ligament and hinge teeth. The mantle lobes secrete the valves, and the mantle crest creates the other parts.

The mantle itself is attached to the shell by numerous small mantle retractor muscles, which are arranged in a narrow line along the length of the interior of the shell. The position of this line is often quite clearly visible on the inside of each valve of a bivalve shell, as a shiny line, the pallial line, which runs along a small distance in from the outer edge of each valve, usually joining the anterior adductor muscle scar to the posterior adductor muscle scar. The two adductor muscles are what allow the bivalve to close the shell tightly.

In some bivalves the mantle edges fuse to form siphons, which take in and expel water during suspension feeding. Species which live buried in sediment usually have long siphons, and when the bivalve needs to close its shell, these siphons retract into a pocket-like space in the mantle. This feature of the internal anatomy of a bivalve is clearly indicated on the interior of the shell surface as a pallial sinus, an indentation in the pallial line.

The valves of the shell are made of either calcite (as with, e.g. oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer, as with the pterioida which often have this layer in the form of nacre or mother of pearl. The outermost layer of the shell is the periostracum, which is composed of a horny organic substance. This forms a yellowish or brownish "skin" on the outside of the shell.The periostracum may start to peel off when it is allowed to dry out for long periods.

The shell is added to, and increases in size, in two ways - by increments added to the open edge of the shell, and by a gradual thickening throughout the animal's life.

The two shell valves are held together at the animal's dorsum by the ligament, which is composed of the tensilium and resilium. The ligament opens the shells.

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