Mortensen's Cockle or Atlantic Giant Cockle
Phylum: Mollusca, Class: Bivalvia, Order: Cardiida, Family: Cardiidae, Genus: Dinocardium, Species: Dinocardium robustum · Cardiidae (Cockle family) · Bivalve (two-shelled), whole shell, fan or heart-shaped when viewed from the side
Species
Dinocardium robustum
Shell Type
Bivalve (two-shelled), whole shell, fan or heart-shaped when viewed from the side
Family
Cardiidae (Cockle family)
Size
Approximately 3 inches in length and width, which is a medium size for this species. Adults typically range from 2 to 5 inches.
Color & Pattern
The visible exterior is heavily sun-bleached and worn, presenting a uniform chalky off-white or cream color. Any original reddish-brown markings or zigzag patterns characteristic of live specimens have been completely faded by the sun and surf.
Rarity
Common
Habitat
Shallow subtidal and intertidal zones, typically found buried just beneath the surface of clean sand in high-energy marine environments along open coastlines.
Geographic Range
Western Atlantic Ocean, ranging from Virginia down through Florida, the Gulf of Mexico, and the Caribbean.
Description
A sturdy, robust bivalve shell featuring prominent, regular radial ribs radiating from the umbo to the margin. The shell has a rounded, inflated profile with a scalloped edge along the ventral margin, giving it a classic heart-like cross-section.
Key Features
Heavy, solid construction with strong, rounded radial ribs (typically around 32 to 36 ribs), asymmetrical shape with a pronounced beak or umbo, and a finely toothed inner margin.
Collector Value
Low to moderate commercial value due to its common occurrence, though pristine specimens with vibrant natural patterns are highly sought after by casual beachcombers.
Condition Notes
Fair condition. The shell is structurally intact as a single valve but exhibits significant weathering, sun-bleaching, and minor surface pitting or boring holes from marine organisms. Original coloration is lost.
Interesting Facts
Cockles are capable of rapid movement using a muscular, axe-shaped foot, which they use not only to burrow into the sand but also to leap away from approaching predators like starfish.
Ecological Role
Serves as an important filter feeder, helping to clear particulate matter from the water column, and acts as a vital food source for various marine predators including rays, large fish, and carnivorous gastropods.
Similar Species
Atlantic Strawberry Cockle (Americardia media), which is generally smaller, more square-profiled, and features brighter reddish spots and prominent spines on the ribs.
Beach Finding Tips
Look along high-energy sandy ocean beaches after storms or at low tide, scanning the wrack line where whole valves and fragments often wash ashore.