Ocean Quahog (also known as Black Clam, Mahogany Clam)
Phylum: Mollusca, Class: Bivalvia, Order: Venerida, Family: Arcticidae, Genus: Arctica, Species: Arctica islandica · Arcticidae (Ocean Quahog family) · Bivalve (two-shelled clam), whole weathered sub-fossil valves, rounded triangular to oval shape
Species
Arctica islandica
Shell Type
Bivalve (two-shelled clam), whole weathered sub-fossil valves, rounded triangular to oval shape
Family
Arcticidae (Ocean Quahog family)
Size
The central large specimen is approximately 4 to 5 inches in width, which is large and typical for mature adults of this long-lived species. Adults generally range from 2 to 4.5 inches.
Color & Pattern
The specimens shown are heavily weathered, sun-bleached, and chalky white to pale gray, having lost their natural glossy dark brown to black periostracum. Interior surfaces are not visible, but typically feature a white porcelain-like finish.
Rarity
Common in deep waters and commercial catches, but beach-cast whole adult specimens of this size in weathered sub-fossil condition are moderately uncommon depending on the local coastline.
Habitat
Found buried in muddy sand or fine gravel substrates in cold, sublittoral to deep offshore waters, typically ranging from shallow depths down to several hundred meters.
Geographic Range
Boreal to subarctic waters of the North Atlantic Ocean, extending from the Arctic down to Cape Hatteras in North America, and along the European coasts from Iceland and Norway to the Bay of Biscay.
Description
These robust, heavy bivalve shells exhibit a rounded-oval or heart-shaped profile when viewed from the side. They feature prominent, deeply etched concentric growth lines spanning the exterior surface, highlighting decades of slow growth. The umbo is slightly eccentric and curved forward. Due to exposure and weathering, the outer surfaces have lost their original organic coating, leaving a heavily chalky and porous texture.
Key Features
Heavy, solid construction, rounded-oval outline, absence of a pallial sinus (distinguishing it from similar large clams like the surf clam), and pronounced coarse concentric growth ridges.
Collector Value
Low commercial value due to abundance, though exceptionally old specimens or pristine whole pairs with intact periostracum are prized by marine biologists and specialized shell collectors.
Condition Notes
Fair to Good condition for beach-combed or sub-fossil specimens. They show significant weathering, chalkiness, and surface pitting with complete loss of periostracum. Some specimens show structural fractures or surface erosion, typical of specimens tumbled in marine environments.
Interesting Facts
Arctica islandica holds the record for being one of the longest-lived animals on Earth. A famous specimen named 'Ming' was found to be 507 years old, making it older than the Ming Dynasty when it was collected.
Ecological Role
Plays an important role in benthic ecology by bioturbating sediments and serving as a food source for bottom-feeding fish and predatory invertebrates. Their long lifespan makes them invaluable for climate and marine history research via sclerochronology.
Similar Species
Hard Clam or Quahog (Mercenaria mercenaria), which can be distinguished by the presence of a distinct pallial sinus on the interior and a flatter shape; and Surf Clams (Spisula), which possess a different hinge structure.
Beach Finding Tips
Best found along North Atlantic and northern European beaches following severe winter storms, heavy wave action, or dredging operations that dislodge offshore benthic sediments.