Giant Keyhole Limpet
Megathura crenulata


Physical Description

  • Slimy gray, black, or brown covering a gray oblong, oval shell
  • A black central hole at the top of the shell, the portal through which waste products are released
  • Sessile, attached to rocks


  • Monterey, California to Baja California, Mexico


  • Intertidal zone
  • Depths up to 108 feet


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


  • 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;

Photo: Frederic Pache

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