Ocean Quahog (also known as Quahog, Black Clam, Mahogany Clam)
Phylum: Mollusca | Class: Bivalvia | Order: Venerida | Family: Arcticidae | Genus: Arctica | Species: Arctica islandica · Arcticidae (Ocean Quahog family) · Bivalve (single valve shown, disarticulated); rounded to thick oval shell shape
Species
Arctica islandica
Shell Type
Bivalve (single valve shown, disarticulated); rounded to thick oval shell shape
Family
Arcticidae (Ocean Quahog family)
Size
Approximately 3.5 to 4 inches (9-10 cm) in length. Adults typically range from 2.5 to 5 inches (6 to 13 cm).
Color & Pattern
Dark brownish-black to grey periostracum outer layer covering a solid white calcium carbonate shell layer beneath. Fine, closely spaced concentric growth rings extend outward from the prominent umbo. Some chips reveal the chalky white interior layer.
Rarity
Common in deep offshore waters, though complete beach-cast specimens with intact dark periostracum are moderately uncommon to rare depending on local wave energy.
Habitat
Benthic marine environments in cold temperate to subarctic waters, typically burrowing in sandy to muddy sediments at depths of 30 to 800 feet (10 to 250 meters).
Geographic Range
North Atlantic Ocean, ranging from the East Coast of North America (Newfoundland south to North Carolina) to Northern Europe, the Baltic Sea, and around Iceland.
Description
A robust, heavy single valve of Arctica islandica exhibiting a rounded, slightly sub-trigonal outline. The exterior surface is densely covered in fine concentric growth lines and covered by a dark, charcoal-grey to dark brown fibrous periostracum. The umbo is pronounced and directed anteriorly. The margins are smooth, thick, and well-preserved.
Key Features
Distinctive dark brown to black periostracum, thick and heavy rounded valve, fine concentric growth lines, smooth inner shell margins without serrations, and a well-developed cardinal hinge structure.
Collector Value
Low to moderate commercial value ($3–$15 depending on preservation and size), but highly valued by collectors and scientists due to its remarkable longevity and paleoclimatological importance.
Condition Notes
Good overall condition for a beachcombed valve. Shows minor mechanical wear and small flakes off the dark periostracum near the umbo revealing the white chalky layer underneath, but retains excellent overall structural integrity and clear growth rings.
Interesting Facts
Arctica islandica is one of the longest-living animals on Earth, with some individuals verified to live well over 500 years. Growth rings on their shells are studied like tree rings (sclerochronology) to reconstruct historic ocean temperature and climate changes.
Ecological Role
Infaunal filter feeder that burrows into sandy-mud ocean bottoms, pumping water to feed on phytoplankton and organic detritus. Serves as important prey for bottom-feeding fish, crabs, and sea stars, though adults burrow deeply for protection.
Similar Species
Mercenaria mercenaria (Hard Clam / Northern Quahog) which lacks the persistent black periostracum and typically features purple interior staining and fine margin serrations; Arctica islandica has smooth inner margins.
Beach Finding Tips
Best found along northern Atlantic beaches after winter storms or heavy tides, particularly in New England, Atlantic Canada, and northern European shores where deep bottom sediments are disturbed.