Keyhole Urchin, also known as the Five-Hole Keyhole Urchin or Sea Biscuit
Phylum: Echinodermata, Class: Echinoidea, Order: Clypeasteroida, Family: Mellitidae, Genus: Mellita, Species: Mellita quinquiesperforata · Mellitidae (Keyhole Urchin and Sand Dollar family) · Echinoid test (endoskeleton of a marine invertebrate, commonly referred to as a shell)
Species
Mellita quinquiesperforata
Shell Type
Echinoid test (endoskeleton of a marine invertebrate, commonly referred to as a shell)
Family
Mellitidae (Keyhole Urchin and Sand Dollar family)
Size
Approximately 3.5 inches in diameter, which is typical for an adult of this species. Adults generally range from 2 to 5 inches.
Color & Pattern
Chalky white to pale off-white, completely sun-bleached with no remaining velvety purple or greenish-brown live coloration. The dorsal surface features a delicate, etched petaloid (flower-like) pattern with five radiating petals, along with five distinct elongated lunules (keyholes).
Rarity
Common, frequently found washed up on sandy beaches throughout its geographic range after storms or high tides.
Habitat
Found in shallow marine waters, typically in intertidal to subtidal zones down to about 30 meters. Prefers soft sandy bottoms, sandbars, and protected bays where they can partially bury themselves.
Geographic Range
Western Atlantic Ocean, ranging from Cape Cod, Massachusetts, down through the Gulf of Mexico, the Caribbean, and south to Brazil.
Description
A remarkably flattened, disc-like echinoid test characterized by its signature five keyhole-like slots (lunules) piercing through the skeleton. The aboral surface displays a pristine, sharply defined central petalloid pattern used for respiration. The edges are gently rounded, and the overall structure is remarkably delicate yet symmetrical.
Key Features
The presence of exactly five distinct, elongated slots (lunules) that pass completely through the test, combined with the flattened, disc-shaped profile and the central five-petaled star (petalloid) design.
Collector Value
Low to moderate commercial value as they are quite common, typically valued between $1 to $5. They are heavily sought after by casual beachcombers and used extensively in coastal crafts and home decor.
Condition Notes
Very good collectibility grade, likely Fine to Gem condition for a beach-found test. It is intact with minimal edge chipping and all five lunules well-formed, though it is thoroughly bleached white from sun exposure.
Interesting Facts
Unlike snails or bivalves, keyhole urchins are closely related to sea urchins and starfish. The slots, or lunules, help the living animal anchor itself in shifting sands against strong waves and allow sand to pass over the top during feeding.
Ecological Role
Keyhole urchins play a vital role in marine benthic ecosystems by bioturbating (churning and aerating) sandy sediments as they burrow and feed on organic detritus, microscopic algae, and diatoms.
Similar Species
The Six-Keyhole Urchin (Mellitaiesitata) which possesses six lunules instead of five, and the common Common Sand Dollar (Echinarachnius parma), which lacks keyhole slots entirely.
Beach Finding Tips
Look along sandy ocean beaches and sandbars at low tide, particularly early in the morning or following rough sea storms. Gently sift through the driftlines where light shells and tests accumulate.