Keyhole Urchin / Five-hole Keyhole Urchin / Sand Dollar
Phylum: Echinodermata, Class: Echinoidea, Order: Clypeasteroida, Family: Mellitidae, Genus: Mellita, Species: Mellita quinquiesperforata · Mellitidae (Keyhole Urchin family) · Echinoid test (endoskeleton, commonly referred to as a sea urchin or sand dollar test)
Species
Mellita quinquiesperforata
Shell Type
Echinoid test (endoskeleton, commonly referred to as a sea urchin or sand dollar test)
Family
Mellitidae (Keyhole Urchin family)
Size
Approximately 3.5 to 4 inches in diameter, which is typical for a mature adult specimen of this species. Adults generally range from 2 to 5 inches.
Color & Pattern
Chalky white to pale off-white, resulting from sun-bleaching and weathering after the animal's death. When alive, the test is covered in a dense velvety layer of tiny purple, reddish-brown, or greenish spines. The upper surface features a distinct, flower-like petaloid pattern composed of five pores.
Rarity
Common to Very Common. Frequently found washed up on sandy beaches throughout its natural range, especially after storms.
Habitat
Found in shallow marine waters, typically in intertidal to subtidal zones down to depths of about 30 feet. They prefer sandy or muddy bottoms, often half-buried in sand flats or in coastal seagrass beds.
Geographic Range
Western Atlantic Ocean, extending from Cape Hatteras, North Carolina, southward through the Gulf of Mexico, the Caribbean, and along the coast of South America to Brazil.
Description
A flat, disc-shaped calcareous endoskeleton featuring a radial symmetry. The upper surface displays a clear, embossed five-petaled flower (petaloid) design used for respiration. Most notably, this specimen contains five distinct elongated lunules or 'keyholes' piercing completely through the test, including one prominent posterior anal lunule.
Key Features
The presence of exactly five elongated oval slots (lunules) piercing the flat, rounded-to-pentagonal disc, distinguishing the genus Mellita from solid sand dollars and other urchin families.
Collector Value
Low to Moderate commercial value, but highly popular among casual beachcombers and nature crafters. Specimens are inexpensive and widely available, though collectors prize large, flawless, unbroken tests.
Condition Notes
Good to Fine condition. The test appears largely intact with minor wear along the fragile outer margins. The central petaloid pattern is crisp and clearly visible. The chalky white color indicates it is a fully bleached beach-worn test devoid of organic material.
Interesting Facts
Living sand dollars use tiny cilia and movable spines to crawl across the seafloor and burrow into the sand. The keyholes in keyhole urchins help prevent the flat animal from being dislodged or flipped over by strong wave action by allowing water to flow directly through the test.
Ecological Role
Keyhole urchins play an important role in marine benthic ecosystems by ingesting organic detritus and microalgae from sand grains, helping to aerate and rework the sandy substrate. They serve as a food source for bottom-feeding fish, crabs, and sea stars.
Similar Species
Five-rayed Sand Dollar (Echinarachnius parma) which lacks the keyhole slots; other Mellita species such as Mellita isometra, which can be distinguished by subtle differences in the placement and shape of the lunules.
Beach Finding Tips
Look along gently sloping, sandy ocean beaches at low tide, particularly early in the morning or immediately following a rough storm when wave action deposits lightweight tests onto the shoreline.