Butter Clam

This large clam can live for more than twenty years and grow to a length of 15 cm (6 in), with smaller individuals being nearly as high as they are long. The umbones are set at an angle of more than 110° and the ligament joining the valves is black and external. There are no radial ridges but the oval valves are sculpted by well-defined concentric rings. Each valve has three cardinal teeth. The general color of the exterior of the shell is white, but this may be stained reddish-brown by iron sulfide from the sediment. The interior of the shell is smooth and white, but not glossy, with a pallial line, a pallial sinus and two adductor muscle scars of equal size. The siphons are united and about 4 cm (1.6 in) long, and have black tips.

S. gigantea is native to shallow waters in the northeastern Pacific Ocean. Its range extends from the Bering Sea and the Aleutian Islands southwards to San Francisco Bay in California, although it rarely occurs south of Humboldt Bay. It is found buried in soft sediments such as sand, muddy sand and gravel, from the low intertidal zone down to about 40 m (130 ft).

This clam lives buried in soft sediments, sometimes burrowing as deep as 35 cm (14 in) below the surface. It is a filter feeder and when feeding it extends its siphons to draw in a respiratory current of water, removes the phytoplankton and other planktonic organic particles, and expels the remaining water. Small pea crabs such as Pinnixa faba sometimes live symbiotically inside the mantle cavity, and the clam is preyed on by the larger Dungeness crab, the Lewis's moon snail, sunflower sea stars, mottled stars and sea otters.