Bivalvia - Anatomy - Mantle and Shell

Mantle and Shell

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 the periostracum, 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. Although the (sometimes faint) concentric rings on the exterior of a valve are commonly described as "growth rings" or "growth lines", a more accurate method for determining the age of a shell is by cutting a cross section through it and examining the incremental growth bands. Use of this technique has changed views on the longevity of many bivalves. For example, the soft-shell clam (Mya arenaria) was thought to be short-lived, but has now been shown to have a lifespan of at least 28 years.

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.