Giant Keyhole Limpet
Megathura crenulata

Physical Description

• A large gray oblong, oval shell with a single black central hole at the top of the shell, the portal through which waste products are released.
• Gray, black, or brown slimy mantle (body wall) covering the shell, unlike any other limpet that has mantle under the shell.
• Adult shell is 3-5 inches in diameter.
• Sessile animal, permanently attached to rocks.

Range

• Monterey, California to Baja California, Mexico.

Habitat

• Live in the intertidal zone.
• Found at depths up to 108 feet.

Reproduction

• Reproduce by broadcast spawning.
• Embryos develop into planktonic larvae and then juvenile veligers (tiny larvae with cilia to move in water) before settling somewhere permanently and growing into adult form.

Diet

• Filamentous cyanobacteria, diatoms, brown and red algae such as seaweed, seagrass, foraminifera, hydrozoans, bryozoans, nematodes, bivalves, gastropods, crustaceans, and tunicates.

Predators

• People use this species for medicinal uses.
• The primary predators for limpets are sea stars, birds, and the occasional crab.

Interesting Facts

• This species is one of the largest keyhole limpets.
• The whole on the top of the shell makes them different from true limpets, which release waste from the mantle beneath the shell. 
• The blood of this species is used in many cancer treatments and vaccines, as it stimulates the immune system and its protein transports molecules throughout the body.  
• A liter of blood from a keyhole limpet will produce 20 grams of protein, which can be worth as much as $100,000.
• Their shells were used as currency among Native Americans.

Sources: Harris and Markl, 1999; Harvesting blood from limpets for a cancer vaccine; How mollusk blood could cure cancer; Marine Biology. 8th ed.; Encyclopedia of Life; CaliforniaTidePools.com; sealifebase

Photo: Frederic Pache