Keyhole Limpet (specifically the Cayennne Keyhole Limpet or similar Fissurellid)
Phylum: Mollusca, Class: Gastropoda, Order: Lepetellida, Family: Fissurellidae, Genus: Diodora, Species: Diodora cayenensis · Fissurellidae (Keyhole Limpet family) · Gastropod (univalve); Cap-shaped/conical with an apical perforation (hole). Both appear to be whole shells showing the dorsal (exterior) view.
Species
Diodora cayenensis (likely)
Shell Type
Gastropod (univalve); Cap-shaped/conical with an apical perforation (hole). Both appear to be whole shells showing the dorsal (exterior) view.
Family
Fissurellidae (Keyhole Limpet family)
Size
Large specimen approx. 20-25mm, smaller approx. 12-15mm. This is standard for the species, which typically grows to 1-2 inches (25-50mm).
Color & Pattern
Base color is off-white to creamy grey. They feature a distinct dark greyish-blue or violet-tinted ring around the central hole. Radiating brown or tan stripes/spots extend from the center to the margin. The coloration appears naturally weathered, common for beach-worn specimens.
Rarity
Common to Very Common. These are frequently found by beachcombers along the Atlantic and Gulf coasts of the Americas.
Habitat
Marine; intertidal and shallow subtidal zones. They prefer attachment to hard substrates like rocks, jetty stones, or coral rubble where they graze on algae.
Geographic Range
Commonly found in the Western Atlantic, ranging from New Jersey south through Florida, the Gulf of Mexico, Caribbean, and down to Brazil.
Description
A low-profile, cap-shaped shell characterized by its roughly oval footprint and a small hole at the apex (the 'keyhole'). The sculpture consists of numerous fine radial ribs intersected by concentric growth lines, creating a slightly latticed or beaded texture. The margin is finely crenulated or scalloped.
Key Features
The most diagnostic feature is the sub-central aperture (hole) at the peak of the shell. In Diodora species, the internal callus surrounding the hole is truncated (squared off) on one side, distinguishing it from the genus Lucapina.
Collector Value
Low monetary value (approx. $0.50 - $2.00 per specimen). They are valued by beginners and craft-makers for their unique shape and the natural hole which allows them to be easily strung for jewelry.
Condition Notes
Beach-worn condition. The apex hole is intact, but the fine radial ribbing has been somewhat smoothed by wave action. The color remains visible but is slightly faded. Overall grade: Good (collectible for general interest/education).
Interesting Facts
The 'keyhole' serves a vital biological purpose: it acts as an exhaust port for water. Water enters the mantle cavity through the front, passes over the gills, and exits through the hole, carrying waste and CO2 away from the animal's head.
Ecological Role
Herbivorous grazers that control algal growth on reefs and rocky shores. They are prey for crabs, predatory sea snails, and shorebirds.
Similar Species
Lucapina suffusa (Finely-rayed Keyhole Limpet), which is flatter and more elongated, and Fissurella barbadensis (Barbados Keyhole Limpet), which typically has more robust ribs.
Beach Finding Tips
Look near the high tide line among shell drift or explore rock pools at low tide. They are often found in areas with heavy shell rubble or near man-made rock jetties.