Variable Coquina, Butterfly Shell, Bean Clam
Phylum: Mollusca, Class: Bivalvia, Order: Cardiida, Family: Donacidae, Genus: Donax, Species: Donax variabilis · Donacidae (Wedge Clams or Coquina family) · Bivalve (Single Valve). Classification: Wedge-shaped or triangular.
Species
Donax variabilis
Shell Type
Bivalve (Single Valve). Classification: Wedge-shaped or triangular.
Family
Donacidae (Wedge Clams or Coquina family)
Size
Approximately 0.5 to 0.75 inches (12-19mm). This is a mature adult size for this species; they rarely exceed 1 inch.
Color & Pattern
Muted lavender-gray to dusky purple with concentric growth bands. Some radial striping is visible near the posterior end. Inner surface typically reflects the exterior color with a darker purple pigment near the hinge and muscle scars. Surface appears slightly glossy despite some weathering.
Rarity
Very Common. These are among the most abundant shells found on Atlantic and Gulf Coast beaches, often appearing by the thousands in the swash line.
Habitat
Found in the intertidal zone of high-energy sandy beaches. These clams live in the 'wash' zone, where they use a muscular foot to rapidly burrow into the sand as waves recede.
Geographic Range
Western Atlantic, ranging from Virginia south to Florida, and throughout the Gulf of Mexico to Texas and Mexico.
Description
A small, sturdy, wedge-shaped bivalve with one end broadly rounded and the other shorter and more pointed. The surface is characterized by fine concentric rings and very fine radial ribs that are most prominent at the posterior end. When both valves are found joined together, they resemble butterfly wings, giving them their popular nickname.
Key Features
Distinctive wedge-like or 'toe' shape, fine cancellate sculpture (the intersection of radial and concentric lines), and an incredibly diverse range of possible colors (though this specimen is the purple-gray morph).
Collector Value
Low monetary value ($0.10 - $0.50) due to extreme abundance. However, they are highly desired by hobbyists for 'color series' collections because they come in nearly every color of the rainbow, including yellow, orange, pink, and white.
Condition Notes
Good. The valve is intact with no major chips at the margin. The color is slightly dull, indicating some time spent on the beach exposed to the sun, but the structural integrity is excellent. Collectibility grade: Fine.
Interesting Facts
Coquinas are famous for their 'dancing' behavior; as waves wash over them, they emerge from the sand and re-bury themselves in seconds to stay in the ideal feeding zone. Historically, they were used in Florida to make 'Coquina broth' and were a primary building material (as coquina rock) for the Castillo de San Marcos in St. Augustine.
Ecological Role
Vital part of the beach ecosystem; they are active filter feeders that clarify water and serve as a primary food source for shorebirds (like Sanderlings), crabs, and many species of fish.
Similar Species
Donax fossor (strikingly similar but usually smaller and found further north) and Donax texasiana (specific to the western Gulf).
Beach Finding Tips
Look at the very edge of the water as waves retreat. You will often see them 'pop' out of the sand and dig back in. To find empty shells, check the high-tide line or 'shell hash' piles where smaller specimens settle.