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.
Phylogeny: The Giant Keyhole Limpet, Megathura crenulata (G.B. Sowerby I, 1825), is a gastropod member of the Fissurellidae Family of Keyhole Limpets. The genus Megathura is one of forty-five genera in this family, and this is the only species in this genus. This species is known in Mexico as Abulón Chino and Fissurella Gigante.
Morphology: As gastropods, Giant Keyhole Limpets have a muscular foot, two upper, and two lower tentacles, eyes located at the tips of the upper tentacles, and a mouth with special rasp-like feeding organ called the radula. The mantle may cover almost the entire shell, leaving only a small area around the keyhole uncovered. The mantle may be black, beige, or mottled gray in color. The Giant Keyhole Limpet shell is oval in outline and has fairly low profile. The exterior is sculpted by numerous fine radial ridges and even finer concentric ridges. The keyhole is large, ovate, and 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. Giant Keyhole Limpet Shells reach a maximum of 14.0 cm (5.5 inches) in length and 8.9 cm (3.5 inches) in height.
Habitat and Distribution: Giant Keyhole Limpets use their strong, muscular, foot to cling to rocks and are found in the intertidal zone, and to depths up to 34 m (110 feet). They are a temperate to subtropical Eastern Pacific species that have a limited range in Mexican waters of the Pacific Ocean being found along the west coast of the Baja Peninsula from the California border south to Bahia Asuncion, Baja California Sur.
Diet: Giant Keyhole Limpets are opportunistic omnivores that eat a diversity of organisms, including brown and green algae, bivalves, bryozoans, crustaceans, cyanobacteria, diatoms, gastropods, hydrozoans, nematodes, and tunicates.
Predators: Giant Keyhole Limpets are preyed upon by shore birds, fish, marine mammals, and sea stars.
Reproduction: n Giant Keyhole Limpets are gonochoric and reproduce sexually through broadcast spawning, with external fertilization.
Ecosystem Interactions: Any form of commensal, parasitic or symbiotic relationship of the Giant Keyhole Limpet has not been documented.
Human Interactions: Giant Keyhole Limpets are edible and occasionally targeted by artisanal fisheries. Historically, their shells were highly valued as items of social status and used in ritual practices of native populations. In modern times, this species is of interest medically as Giant Keyhole Limpets are a source of hemocyanin, which is used as a carrier of antibodies for research and therapeutic applications. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.
Synonyms: Fissurella crenulata and Lucapina crenulata.