Coquina, Wedge Shell, Butterfly Shell
Phylum: Mollusca, Class: Bivalvia, Order: Cardiida, Family: Donacidae, Genus: Donax, Species: Donax variabilis · Donacidae (Wedge Clam family) · Bivalve (two-shelled). Specimen is a single valve (one half), wedge-shaped and slightly convex.
Species
Donax variabilis (65% confidence; specimen is heavily eroded)
Shell Type
Bivalve (two-shelled). Specimen is a single valve (one half), wedge-shaped and slightly convex.
Family
Donacidae (Wedge Clam family)
Size
Approximately 12-18mm in length. This is typical for an adult Donax variabilis, which generally reaches a maximum of 25mm.
Color & Pattern
Base color appears creamy white to pale tan. The natural vibrant bands of purple, yellow, or pink are significantly faded due to beach erosion and sun bleaching. The interior is not visible, though usually it features a polished, glossy sheen.
Rarity
Very Common. These are among the most abundant shells found on Atlantic and Gulf coast beaches, often found in "wash-ups" containing thousands of valves.
Habitat
Intertidal zone of sandy beaches. These animals live in the "wash zone" or "surf zone," constantly migrating up and down the beach with the changing tides by burrowing rapidly into the sand.
Geographic Range
Western Atlantic, specifically from Virginia, USA, down to the Gulf of Mexico and the Caribbean shores.
Description
A small, sturdy, wedge-shaped bivalve. The shell is inequilateral, with the anterior end longer and more rounded than the steeply sloped posterior end. The surface is relatively smooth with very fine radial strikes (hidden by erosion in this specimen). When fresh, these shells often exhibit a "butterfly" appearance when both valves remain hinged together in the sand.
Key Features
Distinctive wedge-like triangular shape, small size, and a serrated inner margin (though difficult to see here). The posterior end is much shorter and more blunt than the anterior end.
Collector Value
Minimal to Low monetary value. These are generally collected for hobbyist displays, jar fillers, or craft projects rather than as high-end specimen shells. Value increases for 'Butterfly sets' (intact hinges) with rare solid colors like deep violet or bright yellow.
Condition Notes
Fair to Poor condition. The specimen shows significant signs of beach wear, including rounding of the edges and loss of the original lustrous periostracum and vibrant color patterns. It is a single worn valve rather than a paired set.
Interesting Facts
Coquina clams are known for their rapid burrowing ability, utilizing their foot to disappear into the sand in seconds as waves recede. Historically, the shells were used in Florida (specifically St. Augustine) as a construction material called 'Coquina rock' for forts and buildings because the porous nature of the compressed shells could absorb cannonball impacts rather than shattering.
Ecological Role
Primary consumer; they filter phytoplankton from the water. They are a vital food source for shorebirds (like sanderlings), crabs, and fish (like pompano). They serve as excellent bioindicators for beach health.
Similar Species
Donax fossor (which is smaller and found further north) and Donax texasianus (found in the Western Gulf). It can be distinguished from juvenile hard clams by its elongated, asymmetrical wedge shape.
Beach Finding Tips
Look at the water's edge just as a wave recedes. You will often see the live clams quickly digging back into the wet sand. For empty shells, check the 'wrack line' or small tidal pools where debris accumulates during low tide.