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 this hinge area that the clam extends as it grows.  Clams are also hermaphroditic, so as they grow they will change from males (producing sperm) to females (producing eggs).

Scallops

Another type of bivalve, scallops, such as the Atlantic bay scallop, do not burrow in the sand.  Juvenile bay scallops are often found attached to blades of eelgrass (zostera marina) or other sea grasses, with larger animals resting on the bottom.  Scallops can 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.

Scallops are hermaphroditic, in that they produce both eggs and sperm throughout the reproductive stage of their life cycle, which is about half of their approximate two year life span. 

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 sea stars, crabs, and snails.

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.  As they are on or above the surface of the sediment, they simply open their growing shells to filter, using their gills, algae and oxygen from the water. 

Oysters are also bivalves and are hermaphroditic in that they 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.

Shells of our Region

Seashells and Other Things from the Mid-Atlantic Shore

Shells 

	Shellfish Anatomy		Northern Quahog
	Stout Razor Clam		Atlantic Jacknife Clam
	Ponderous Ark		False Angel Wing
	Atlantic Bay Scallop		Eastern Oyster
	Jingle Shells		Atlantic Moon Snail
	Lightning Whelk		Knobbed Whelk
	Channeled Whelk		Slipper Shells

Other Things Often Washed Ashore 

	Skate Egg Case		Whelk Egg Capsules
	Horseshoe Crab		Spider Crab
	Keyhole Sand Dollar		Moon Jellyfish

(Courtesy of the Assateague Naturalist)

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