Coquina Clam (Butterfly Shell, Bean Clam)
Phylum: Mollusca, Class: Bivalvia, Order: Cardiida, Family: Donacidae, Genus: Donax, Species: Donax variabilis · Donacidae (Bean Clam family) · Bivalve (two-shelled, wedge-shaped)
Species
Donax variabilis
Shell Type
Bivalve (two-shelled, wedge-shaped)
Family
Donacidae (Bean Clam family)
Size
Approximately 0.75 inches long, which is a standard adult size for this species. Adults typically range from 0.5 to 1 inch.
Color & Pattern
The prominent shell held in the palm is heavily worn and sun-bleached, exhibiting a uniform off-white to pale cream coloration with faint traces of original concentric growth banding. The natural color variations for this species are famously diverse—ranging from yellow, orange, pink, purple, to blue with radial rays—but beach-worn specimens commonly fade to white.
Rarity
Very Common
Habitat
Found in the intertidal zone of high-energy sandy ocean beaches, where they rapidly burrow into the shifting sands using a muscular foot as waves wash over them.
Geographic Range
Western Atlantic Ocean, from Virginia southwards along the Florida coast, and into the Gulf of Mexico. This specimen specifically originates from Kill Devil Hill beach in the Outer Banks, North Carolina.
Description
A small, wedge-shaped bivalve shell with a smooth, glossy texture when fresh, featuring subtle concentric growth lines. The anterior end is typically elongated and rounded, while the posterior end is shorter and truncated. Due to wave action on high-energy beaches, the specimen shown is a slightly weathered single valve with a smoothed perimeter.
Key Features
Wedge-shaped (cuneate) profile, asymmetrical outline with a short posterior and elongated anterior, smooth exterior surface, and sturdy structure adapted for surviving turbulent surf zones.
Collector Value
Low monetary value due to extreme abundance, but highly cherished by casual beachcombers and crafters who collect them by the handful to make shell art, jewelry, or wind chimes due to their colorful variations.
Condition Notes
Fair to Good condition for a beach-combed specimen. It is a single intact valve rather than a paired living shell. The surface is completely sun-bleached and worn smooth by wave action, lacking the vibrant living colors and periostracum, but structurally solid without major fractures or boring holes.
Interesting Facts
Coquina clams are famous for their mass migrations; they emerge from the sand on the incoming tide to filter-feed on plankton and rapidly re-burrow as the wave recedes. They are a crucial food source for shorebirds, fish, and crabs.
Ecological Role
Coquina clams act as vital biofilters in sandy beach ecosystems, converting suspended organic matter into biomass. They form a critical dietary foundation for coastal shorebirds, game fish, and crustaceans.
Similar Species
Other small bivalves such as marsh clams or tiny tellins, but Donax variabilis is easily distinguished by its distinct wedge shape and habit of living exclusively in high-energy surf-swept sand.
Beach Finding Tips
Look along the wet swash zone of ocean-facing sandy beaches just as a wave recedes. Watch for tiny bubbles or V-shaped depressions in the sand where they are burrowing, or sift through the shell lines deposited at the high tide mark.
Notes
Outer banks, North Carolina on Kill Devils Hill beach