Bivalvia

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Bivalve Biology & Systematics

Bivalves are two-shelled invertebrates belonging to phylum Mollusca and include clams, oysters, mussels and scallops. They are characterized by their very developed gills or "ctenidium" which usually function in biotic respiration and filter feeding. Most bivalves are found burrowing into sand, mud, wood and even rock, although some attach to the surface of hard substrates. Some species are free swimming.

Shell Structure

All bivalves have rigid shells to provide support for their soft bodies. The shell is divided into three parts: the two calcareous valves and the ligament. The ligament is a stiff, organic hinge that holds the two valves together and holds the valves of the shell open when the adductor muscles are relaxed.
The valves are composed of three general layers. The outer most layer, the periostracum, is a thin, paper-like, organic skin on most bivalves. It acts as a base for the other layers to build on and protects the shell from abrasion and dissolution in sea water. The middle layer is called the perismatic layer, and is composed of calcite, a crystaline form of calcium carbonate, which provide a high degree of strength for the shell. The nacre or "mother-or-pearl," on the inner side of the valve, is made of aragonite, a form of calcium carbonate. This smooth, shiny surface protects the animal itself from abrasion against the shell and is the same material that pearls are made from. Nacre is formed around a foreign object, like a grain of sand, that gets lodged between the shell and the mantle so that it won't irritate the mantle.
Both the periostracum and the perismatic layer are secreted by the mantle edge where glands remove calcium carbonate from the animal's blood which has a high concentration of this molecule. The nacre is produced by the entire mantle surface from calcium carbonate in the animal's blood.

Shell External Sculpting
1.Concentric Sculpture (Saxidomus giganteus)

2.Cancelate Sculpture (Protothaca staminea
-Showing evidence of predation by
Polinices lewisii)

3.Radial Sculpture (Clinocardium nuttallii)

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2	3

At the hinge point, within the shell, many bivalves have rounded projections called "teeth." There are two types of teeth in those bivalves that have them: lateral teeth and cardinal teeth. Cardinal teeth are tall and are located close to the umbo and chondrophore. Lateral teeth are long, low and roughly parallel to the umbo and below the cardinal teeth. The teeth help align the valves of the shell so that they close properly. Collectively, the cardinal and hinge teeth are called "hinge teeth".

Although not all bivalves have hinge teeth, they all possess a hinge ligament. The ligament, whether internal, external or partially internal, opens the shell by acting as a hinge spring. The animal thereby expends less energy to keep itself open while feeding and strengthens the connection between the valves.