Keyhole Limpet (also known as Shield Limpet or Volcano Limpet)
Phylum: Mollusca, Class: Gastropoda, Order: Vetigastropoda, Family: Fissurellidae, Genus: Diodora, Species: Diodora aspera · Fissurellidae (Keyhole Limpet family) · Gastropod (univalve, cap-shaped whole shell)
Species
Diodora aspera
Shell Type
Gastropod (univalve, cap-shaped whole shell)
Family
Fissurellidae (Keyhole Limpet family)
Size
Approximately 1 to 1.5 inches in length, which is typical for adult specimens of this species. Adults generally range from 0.75 to 2 inches.
Color & Pattern
The interior shows a glossy white to porcelain surface with hints of pale green and brown shading, while the exterior features alternating grayish-white and dark brownish-gray radial ribs. The colors appear entirely natural and well-preserved.
Rarity
Common
Habitat
Found on rocky intertidal and shallow subtidal zones, typically attached tightly to bare rocks, boulders, or bedrock where they graze on encrusting algae.
Geographic Range
Northeast Pacific Ocean, ranging from the Aleutian Islands in Alaska down to central Baja California, Mexico.
Description
An oval, cap-shaped univalve shell featuring strong, prominent radial ribs crossed by fine concentric growth lines, creating a distinct grid-like or cancellate texture. The margin is finely toothed and scalloped, conforming to the underlying substrate. Though the characteristic apical keyhole is obscured in this particular interior-facing view, the shell displays a smooth, porcelain-like interior margin with a subtle calloused ring surrounding the internal opening.
Key Features
Oval, cone-like cap shape, strong radial ribbing, crenulated margin, and a central or sub-central apical perforation (keyhole) used for water expulsion and waste elimination.
Collector Value
Low to Moderate value. While very popular among regional beachcombers and tidepool enthusiasts, they are common enough that individual specimens generally hold nominal monetary value, typically sold for a few dollars in shell shops.
Condition Notes
Good to Fine condition. The shell is structurally intact with no major chips, cracks, or heavy wear along the scalloped margin. The internal glaze is clear and clean.
Interesting Facts
Keyhole limpets are named for the small opening at the apex of the shell. Water is drawn in under the edge of the shell for respiration and feeding, flows over the gills, and exits through this keyhole along with waste. Unlike true limpets, they are more closely related to abalones.
Ecological Role
As herbivorous grazers, keyhole limpets play an important role in controlling microalgae and algal films on rocky substrates, helping to maintain open space for other sessile marine organisms. They serve as prey for various sea stars, predatory snails, and shorebirds.
Similar Species
Owl Limpet (Lottia gigantea) which lacks the apical keyhole perforation, and other Diodora species such as Diodora cayenensis, which can be distinguished by differences in geographic range and finer sculptural ribbing details.
Beach Finding Tips
Look along rocky shores, jetties, and tidepools during extremely low tides. Search underneath rocks or clinging to vertical rock faces. Freshly dead or washed-up specimens can occasionally be found in shell drift lines along rocky beaches.