Juvenile rock scallops (Hinnites giganteus) are difficult to distinguish from other scallops. They can swim about by flapping their two shells. But as they grow older, they will attach one valve (shell) to a rock and become distorted to fit the rock’s shape.

Rock scallops are prized as food and their thick muscle is extracted and prepared just like scallops.

When washed onto the beach, empty rock scallop shells are easily distinguished by a splash of purple color on the interior, next to the hinge. The outer part of the shell is often riddled with small holes left by an encrusting yellow sponge, which can bore into the calcium shell.

Source: Flotsam, Jetsam and Wrack.