# Giant Keyhole Limpet (*megathura crenulata*) The Giant Keyhole Limpet is a remarkably large marine mollusk native to the rocky shores of the Pacific coast of North America. Unlike typical limpets that rely heavily on a visible shell for protection, this unique gastropod is famous for its thick, velvety mantle that grows up and over its shell, leaving only a small, keyhole-like opening exposed at the top. This opening serves as an exit portal for water, waste, and reproductive cells. In scientific communities, this species is highly celebrated for its contribution to modern medicine. Its copper-based blood contains Keyhole Limpet Hemocyanin, a massive protein that stimulates the human immune system and is used globally in immunology research, diagnostic tests, and the development of cancer vaccines. - **Category:** [Invertebrate](https://www.snappit.app/explore/categories/invertebrate) - **Rarity:** Common - **Danger:** 1/5 · Very low - **Snaps:** 4 - **Habitat:** Found in rocky intertidal and shallow subtidal zones along Pacific coastlines, often clinging to reefs or hidden within dense kelp forests. ![Giant Keyhole Limpet](https://inaturalist-open-data.s3.amazonaws.com/photos/64621018/medium.jpg) Photo: Diego Maldonado de la Torre, CC BY 4.0 · [iNaturalist](https://www.inaturalist.org/photos/64621018) ## Appearance This massive limpet can reach up to 13 centimeters in length, featuring an oval shell with a distinct, keyhole-shaped opening near the center. However, the shell is usually completely covered by its thick, fleshy mantle, which varies in color from dark black and mottled grey to sandy tan. The underside reveals a broad, muscular foot used to grip rocky surfaces tightly. ## Taxonomy - **Kingdom:** Animalia - **Phylum:** Mollusca - **Class:** Gastropoda - **Order:** Lepetellida - **Family:** Fissurellidae - **Genus:** Megathura ## Interesting facts - Its blue-colored blood contains a protein so complex that scientists have been unable to synthesize it in a laboratory, making wild or aquacultured limpets the only source. - The characteristic keyhole at the top of its shell acts like a chimney, allowing wastewater and reproductive materials to exit without getting trapped under the mantle. - Unlike most other limpets that have hard, exposed, cone-shaped shells, the Giant Keyhole Limpet looks and feels more like a large, velvety sea slug due to its expansive mantle. ## Special abilities - **Medical Blood Shield:** Produces Keyhole Limpet Hemocyanin, a highly complex, oxygen-binding protein in its blood that serves as a powerful immunostimulant in human medicine. - **Fleshy Shielding:** Can expand its large mantle to cover and protect its entire shell from predators, preventing invasive organisms from settling on its hard surface. - **Suction Grip:** Uses its wide, muscular foot to create a powerful suction seal on wave-swept rocks, preventing it from being washed away by strong Pacific swells. ## Measurements & details - **Length:** 6-13 cm - **Weight:** 0.1-0.4 kg - **Lifespan:** 5-15 years - **Clutch Size:** 10000-80000 - **Incubation:** 1-3 days ## Diet & Feeding It grazes slowly across rocky substrates, scraping up a variety of organic materials including algae, tunicates, bryozoans, and colonial sponges. - **Primary Foods:** Microalgae, Colonial tunicates, Sponges, Bryozoans - **Foraging Method:** Grazing ## Ecological connections - **Eaten by:** [Ochre Sea Star](https://www.snappit.app/explore/ochre-sea-star) (*Pisaster ochraceus*) — Predator that preys on the limpet using its tube feet to pry it from rocks. - **Hunts:** Orange Puffball Sponge (*Tethya aurantia*) — The limpet grazes on this common tide pool sponge as part of its varied diet. - **Eaten by:** California Two-spot Octopus (*Octopus bimaculoides*) — A common cephalopod predator that hunts the limpet in subtidal zones. ## Safety **Danger:** 1/5 · Very low No special safety notes yet. ## Frequently Asked Questions ### How to identify Giant Keyhole Limpet? Compare the overall shape, opening, spiral, ribs, hinge and surface sculpture. Beach shells often lose the colors and sharp edges they had on the living animal, so shape does most of the work. Seashell Identifier: upload a photo and compare the visible details. ### How long is Giant Keyhole Limpet? 6-13 cm ### How much does Giant Keyhole Limpet weigh? 0.1-0.4 kg ### How long does Giant Keyhole Limpet live? 5-15 years ### What does Giant Keyhole Limpet eat? It grazes slowly across rocky substrates, scraping up a variety of organic materials including algae, tunicates, bryozoans, and colonial sponges. ### Where is Giant Keyhole Limpet usually found? Found in rocky intertidal and shallow subtidal zones along Pacific coastlines, often clinging to reefs or hidden within dense kelp forests. ### How does Giant Keyhole Limpet hunt? Grazing ### How many eggs does Giant Keyhole Limpet lay? 10000-80000 ## Where to spot - [Cabrillo National Monument](https://www.snappit.app/guides/places/cabrillo-national-monument) — California, US · National Park - [Crystal Cove Historic District](https://www.snappit.app/guides/places/crystal-cove-historic-district) — California, US · Historic District - [Sherman Library and Gardens](https://www.snappit.app/guides/places/sherman-library-and-gardens) — California, US · Botanical Garden [View all places to see Giant Keyhole Limpet](https://www.snappit.app/guides/where-to-see/giant-keyhole-limpet) ## More Invertebrate - [Coral](https://www.snappit.app/explore/coral) - [Crab Spider](https://www.snappit.app/explore/crab-spider) (*Mecaphesa asperata*) - [Garden Spider](https://www.snappit.app/explore/garden-spider) (*Araneus diadematus*) - [Brown Recluse Spider](https://www.snappit.app/explore/brown-recluse-spider) (*Loxosceles reclusa*) --- Source: [Giant Keyhole Limpet: Facts & Places to See — Snappit](https://www.snappit.app/explore/giant-keyhole-limpet)