Crepidula perforans Valenciennes 1846

A slipper shell which is a flattened or even concave, irregular cap-shape with an apex directed posteriorly, white, with the free margin of the septum nearly straight, often attached to bivalve shells, especially inside those occupied by hermit crabs. Length up to 5.5 cm but usually not much over 2.5 cm. Variable in shape, conforms to substrate. May be thin and fragile or thick and robust. Periostracum thin, often inconspicuous.

How to Distinguish from Similar Species: Crepidula nummaria has a yellow-brown periostracum. Crepidula fornicata has the apex to the side. Few other species besides this one live inside gastropod shells with hermit crabs.

Occasionally on rock, often inside apertures of shells, inside holes of rock-boring clams.

Geographical Range: Vancouver Island to Baja California

Depth Range: Almost completely subtidal in Puget Sound

Biology/Natural History: In Alaska, is sometimes found on the operculum of the Oregon triton, Fusitron oregonensis. Crepidula nummaria may be synonymous with this species. The systematics needs to be worked out better. The females brood their young, which can distinguish this species from C. nummaria.

Van Damne State Park, Mendocino County, to Palos Verdes Peninsula and San Pedro, Los Angeles County, California. Depth range is poorly documented, but this species seems to be limited to shallow water.