American Oyster (Crassostrea virginica)
Alternate common names: Common Oyster, Eastern
Color: The exterior of the shell is grayish white; the interior is white with a purple muscle scar.
Size: 2 to 6 inches long and 2 inches wide.
Habitat: Hard, rocky, sandy, muddy, or shell-strewn bottoms below the tide line.
Seasonal appearance: All year.
The American oyster is a bivalve mollusk, meaning it has two valves, or shells. The thick, lower valve permanently cements itself to a hard substrate and is shaped differently than the top valve, which is generally smaller and flatter. The hinge connecting the two valves is an elastic cushion held together by a thick, strong muscle. American oysters can provide a great volume of the hard substrate found in estuaries, often aggregating in clusters known as oyster bars.
Life History and Behavior
When they first settle out of the water column, juvenile American oysters are known as spat. The hard surface of the oyster bars on the bottom increases the area available to spat and other organisms upon which to settle. American oysters are preyed upon at various stages in their development by animals including oyster drills, sea stars, fish, crabs, flatworms, oyster catchers, and mud worms.
When filter feeding, their valve muscle relaxes, causing the shell to open, drawing plankton-rich water into the mantle cavity. When the shell is closed, the mantle is filled with sea water, enabling the animal to survive for extended periods out of the water in cold or dry conditions.
Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.