The Assateague Naturalist Information

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Bivalve Anatomy

(Note: Colors have been added to distinguish organs; they are not the natural colors.)

Picture Clams

Clams, such as the northern quahog, are bivalves, meaning that they have shells consisting of two halves, or valves. The valves are joined at the top, and the adductor muscles on each side hold the shell closed. If the adductor muscles are relaxed, the shell is pulled open by ligaments located on each side of the umbo.

The clam's foot is used to dig down into the sand, and a pair of long siphons that extrude from the clams's mantle out the side of the shell reach up to the water above (only the exit points for the siphons are shown). Clams, such as the northern quahog, are filter feeders. Water and food particles are drawn in through one siphon to the gills where tiny, hair-like cilia move the water, and the food is caught in mucus on the gills. From there, the food-mucus mixture is transported along a groove to the palps which push it into the clam's mouth. The second siphon carries away the water. The gills also draw oxygen from the water flow.

The mantle, a thin membrane surrounding the body of the clam, secretes the shell. The oldest part of the clam shell is the umbo, and it is from the hinge area that the clam extends as it grows.

Picture Scallops

Scallops, such as the Atlantic bay scallop, do not burrow in the sand; instead, they lie on the bottom and move by using their adductor muscle to rapidly open and close their valves, ejecting water around the hinge. The adductor muscle is most often eaten, but most of the remainder is also edible.

Around the edge of the scallop's mantle is a series of blue eyes that, though rather weak, can detect movement nearby and warn of the presence of predators, particularly of sea stars.

Picture Oysters

The larvae of oysters, such as the eastern oyster, cement their mantles to rocks, shells, or any other solid objects and spend their lives in one place, opening their growing shells to filter algae from the water.

The oysters change their sex during their lives, starting as males and usually ending as females. The shape of oysters varies and depends mainly on how many crowd about them in the bed as they develop.

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