Coquina Clam (also known as Donax Clam, Butterfly Clam, or Wedge Shell)
Phylum: Mollusca > Class: Bivalvia > Order: Cardiida > Family: Donacidae > Genus: Donax > Species: Donax variabilis · Donacidae (Wedge Clam / Coquina family) · Bivalve (single valve, wedge-shaped)
Species
Donax variabilis (High confidence)
Shell Type
Bivalve (single valve, wedge-shaped)
Family
Donacidae (Wedge Clam / Coquina family)
Size
Approximately 0.5 to 0.75 inches (12-18 mm) wide, which is well within the typical adult size range of 0.5 to 1 inch for this species.
Color & Pattern
Rich reddish-brown to chestnut-orange exterior with fine concentric growth lines. The umbo area shows a lighter, yellowish-tan sun-bleached or eroded patch. Highly glossy finish due to natural shell polish.
Rarity
Very Common (extremely abundant on southeastern US Atlantic beaches, though single intact valves in vivid red or purple shades are prized by beachcombers).
Habitat
Intertidal surf zones on sandy beaches. They live buried shallowly in wet sand right where waves break, rapidly re-burrowing as waves wash over them.
Geographic Range
Western Atlantic ocean from Long Island, New York south through Florida, the Gulf of Mexico, and into the Caribbean. Very common along the North Carolina coastline including Oak Island.
Description
A classic wedge-shaped bivalve valve exhibiting fine concentric striations and a beautiful reddish-amber coloration. The shell features a prominent beak/umbo at the apex, tapering gently toward the ventral margin, with a smooth, glossy surface created by friction with wet beach sand.
Key Features
Distinct asymmetric triangular/wedge shape, smooth exterior with fine concentric growth lines, glossy sheen, and brilliant variable color (yellow, pink, purple, white, brown, or red).
Collector Value
Low monetary value ($0 to $2 for individual shells), but high aesthetic value for craft workers, jar displays, and beachcombers who collect color-sorted sets.
Condition Notes
Good condition. The exterior shows slight erosion near the umbo typical of beachcombed specimens, but retains excellent color intensity, smooth margins, and good overall polish.
Interesting Facts
Living coquinas move up and down the beach with the tide, using a tiny muscular foot to burrow into the sand within seconds after a wave washes over them. When both valves remain connected after dying, they open flat and resemble butterfly wings, giving them the nickname 'Butterfly Clams'. Historically, crushed coquina shells fused into soft limestone were used to build historic fortifications such as Castillo de San Marcos in St. Augustine, Florida.
Ecological Role
Crucial primary consumers in surf-zone food webs. They filter phytoplankton from seawater and serve as a major food source for shorebirds (like sanderlings), crabs, fish (like pompano), and predatory snails.
Similar Species
Donax fossor (which is generally smaller and lacks such intense color variation) and juvenile hard clams (Mercenaria mercenaria, which are rounder and thicker with distinct hinge teeth).
Beach Finding Tips
Look in the swash zone as waves recede on sandy beaches like Oak Island, NC. Watch for tiny bubbles or small, fast-burrowing clusters right at the water's edge at low tide.
Notes
Oak Island Nc