Giant Keyhole Limpet Mollusk and Shell, Megathura crenulata

Giant Keyhole Limpet Mollusk and Shell, Megathura crenulata. Underwater photograph taken in coastal waters of the greater San Diego, California area, May 2018. Photograph courtesy of Bob Hillis, Ivins, Utah.

Giant Keyhole Limpet Shell, Megathura crenulata.  Shell collected off the beach in the greater San Diego, California area, May 2018. Size: 10.1 cm (4.0 inches) x 6.4 cm (2.5 inches). Collection, photograph and identification courtesy of Bob Hillis, Ivins, Utah.

Giant Keyhole Limpet Mollusk and Shell, Megathura crenulata. Underwater photo taken in Mission Bay, San Diego, CA. July 2024. Photograph by Bob Hillis, Ivins, Utah.

The Giant Keyhole Limpet, Megathura crenulata (G.B. Sowerby I, 1825), is a gastropod gastropod member of the Fissurellidae Family of Keyhole Limpets. They are also known in Mexico as fissurella gigante. The shell has an oval fairly low profile, with the exterior being sculpted by numerous fine radial ridges and even finer concentric ridges and the keyhole is large, moth and is slightly forward of center. The exterior of the shell is pink, light reddish-brown or tan; the area around the keyhole and the interior are white. Living specimens may be almost entirely covered by a thick mantle that is black or mottled gray in color. The Giant Keyhole Limpet Shells reach a maximum of 14.0 cm (5.5 inches) in length and 8.9 cm (3.5 inches).

Giant Keyhole Limpets are found clinging to rocks in the intertidal zone to depths up to 34 m (110 feet). They range from Central California to Bahia Asuncion, Baja California Sur. They are of interest medically as they are a source of hemocyanin, which is used as a carrier of antibodies for research and therapeutic applications.