Variable Coquina, Butterfly Shell, Wedge Shell
Phylum: Mollusca, Class: Bivalvia, Order: Cardiida, Family: Donacidae, Genus: Donax, Species: variabilis · Donacidae (Wedge Clam or Coquina family) · Bivalve (Single valve/half-shell), Wedge-shaped/Triangular
Species
Donax variabilis (Confidence: 98%)
Shell Type
Bivalve (Single valve/half-shell), Wedge-shaped/Triangular
Family
Donacidae (Wedge Clam or Coquina family)
Size
Approximately 0.75 inches (19mm) in length. This is a mature adult specimen, as typical sizes for this species range from 0.5 to 1 inch.
Color & Pattern
Highly distinctive concentric banding with a 'plaid' or 'calico' effect. Features alternating bands of slate gray, cream, and warm rust-orange. Fine radial lines intersect the growth rings to create a delicate checkered texture. The interior would likely be a polished purple or white, though not visible here.
Rarity
Very Common. These are among the most abundant shells found on Atlantic and Gulf Coast beaches, often appearing by the thousands.
Habitat
Intertidal zone of high-energy sandy beaches. These animals live in the wash zone where waves break, burrowing rapidly into the saturated sand between wave cycles.
Geographic Range
Western Atlantic, ranging from Virginia south to Florida and the Gulf of Mexico through Texas and Mexico.
Description
A classic 'wedge-shaped' bivalve characterize by a starkly asymmetrical profile. The anterior end is long and rounded, while the posterior end is short and steeply angled. The surface is smooth but marked by incredibly fine radial ribs and concentric growth lines that create a subtle matte texture. The valve is thin but sturdy, designed to withstand the crushing forces of turbulent surf.
Key Features
Triangular/wedge shape with the off-center beak (umbo) near the posterior end. The combination of concentric color bands and fine radial striations is diagnostic of the genus Donax.
Collector Value
Minimal monetary value (typically less than $1) due to extreme abundance. However, they are highly desired by hobbyists for 'Butterfly' displays (re-hinging two matching valves) and are popular in craft work due to their incredible variety of colors.
Condition Notes
The shell is in Fine condition. It is a single valve (unhinged). The growth lines and color bands are sharp and well-preserved, showing minimal beach-wear or sun bleaching. There are no obvious predator bore-holes.
Interesting Facts
Coquinas are famous for their 'migratory' behavior; they use their muscular foot to pop out of the sand and ride waves up and down the beach to stay in the ideal wash zone as the tide changes. Historically, they were harvested by early Florida settlers to make 'Coquina Broth.'
Ecological Role
Critical primary consumer and filter feeder in the surf zone. They serve as a major food source for shorebirds (like Sanderlings), crabs, and surf fish. They are sensitive to beach renourishment projects which can bury their colonies.
Similar Species
Donax fossor (tends to be smaller and less colorful, found further north) and Donax roemeri. It is distinguished from the False Coquina (Heterodonax bimaculatus) which is more oval and less wedge-shaped.
Beach Finding Tips
Look in the 'wash' area at mid-to-low tide. Watch for small 'V' shapes in the sand as waves recede—this is the siphons of the live animal. You can find piles of empty shells in the wrack line or at the high-tide mark.