Atlantic Giant Cockle
Phylum: Mollusca, Class: Bivalvia, Order: Cardiida, Family: Cardiidae, Genus: Dinocardium, Species: Dinocardium robustum · Cardiidae (Cockle family) · Bivalve (fragment). Original shape is inflated, heart-shaped (when viewed from the side with both valves joined).
Species
Dinocardium robustum
Shell Type
Bivalve (fragment). Original shape is inflated, heart-shaped (when viewed from the side with both valves joined).
Family
Cardiidae (Cockle family)
Size
The fragment is approximately 1.5 to 2 inches square. Given the rib spacing, it likely belongs to a large adult shell that would have been 3.5 to 5 inches in total length.
Color & Pattern
Shows alternating bands of tan, cream, and brownish-purple. The ribs feature distinct 'zipper' or 'stair-step' color patterns where darker pigments settle in the growth lines. The color is slightly weathered but still retains the species' characteristic earth-toned pallet.
Rarity
Very Common. Whole shells are frequently found on Myrtle Beach, though large intact specimens with both valves joined (a 'heart') are more uncommon.
Habitat
Found in shallow sandy bottoms, typically in the subtidal zone from the low tide mark to depths of about 100 feet. Prefers high-salinity marine environments.
Geographic Range
Western Atlantic from Virginia south to Florida, Gulf of Mexico, and south to Mexico.
Description
This is a wall fragment of a robust, heavily-ribbed bivalve. The specimen displays strong, rounded radial ribs (approximately 32-36 on a whole shell) that are deeply grooved. The texture is smooth but undulating due to the heavy ribbing. This specific piece highlights the cross-section of the shell's structural strength.
Key Features
Distinguished by the large size, the brownish-purple interior staining (partially visible on the top edge), and the rounded, non-spiny radial ribs that are square-ish in cross-section.
Collector Value
Minimal for a fragment. A large, 'Gem' grade matched pair (both valves) can fetch $5–$15. Its value on Myrtle Beach is primarily as a souvenir of the area's most iconic large bivalve.
Condition Notes
Poor/Fragmentary. This is a beach-worn shard rather than a complete specimen. The edges are tumbled by surf action, and the hinge, beak, and margins are missing. Collectibility is low for this specific fragment, though it serves as a good educational representative of the species' texture.
Interesting Facts
The name 'Cockle' comes from the French word 'coquille' (shell). These animals have a powerful, L-shaped foot that allows them to 'jump' across the sandy floor to escape predators like sea stars.
Ecological Role
A suspension feeder that filters nutrients from the water column. They are a primary prey item for whelks, rays, and sea stars. They are not currently considered threatened or protected.
Similar Species
Prickly Cockle (Trachycardium egmontianum) which has sharp scales on the ribs, or the Yellow Cockle (Divalinga quadrisulcata), which is much smaller and lacks the purple-brown banding.
Beach Finding Tips
Look for these after a strong storm or 'nor'easter' on Myrtle Beach. They are heavy shells, so they often settle in the 'shell wash' or 'trough' near the low tide line rather than high up on the dunes.
Notes
Myrtle Beach SC