Giant Rock Scallop

Adults of Crassadoma gigantea have one valve (the right valve) fixed to a base, often a vertical rock face. This valve often becomes deformed to fit the contours of the rock. The left valve is roughly circular but irregular in outline, thick and heavy, with deep radial ribs. Every third or fourth rib is clad with blunt spines that overhang each other. These often get abraded and worn smooth as the scallop ages. There are flaps of shell known as auricles on either side of the straight hinge. The general colour is brown with the inside of the shell being glossy white with a purple patch at the hinge and a large, central adductor muscle scar. Crassadoma gigantea grows to a diameter of 15 centimetres (5.9 in) in the intertidal zone and 25 centimetres (9.8 in) in the subtidal zone.

Crassadoma gigantea is found on the Pacific Coast of North America, from British Columbia south to Baja California and Mexico. It does not move around as an adult but lives on the sea floor. Its favoured habitats include inside crevices and under boulders, or cemented to rock surfaces, corals or man-made structures such as harbour pilings. It is found at depths down to about 80 metres (260 ft).