Smooth Giant Clam / Smooth Bittersweet Clam (also known as Dog Cockle or Giant Trough Shell)
Phylum: Mollusca, Class: Bivalvia, Order: Venerida, Family: Mactridae, Genus: Mactra, Species: Mactra stultorum · Mactridae (Trough Clams) / Glycymerididae (Bittersweet Clams) · Bivalve (single isolated valve). Shape: Sub-triangular to rounded, convex fan-like shell.
Species
Mactra stultorum (or Glycymeris glycymeris depending on hinge structure)
Shell Type
Bivalve (single isolated valve). Shape: Sub-triangular to rounded, convex fan-like shell.
Family
Mactridae (Trough Clams) / Glycymerididae (Bittersweet Clams)
Size
Approximately 2.5 to 3.5 inches (6 to 9 cm) in length based on the surrounding sink ridges. This is well within the typical adult size range of 5 to 10 cm.
Color & Pattern
Warm beige to light orange-tan exterior with subtle radial bands radiating from the umbo. The surface shows faint growth lines. Colors appear somewhat weathered/sun-bleached with moderate erosion revealing tiny pinhole perforations caused by boring sponges.
Rarity
Very Common (frequently found washed up on sandy beaches along Atlantic and Mediterranean coasts).
Habitat
Found in shallow marine intertidal to subtidal sandy and muddy sea floors, burrowing just beneath the sand surface.
Geographic Range
Common along the coasts of Western Europe, the Mediterranean Sea, and the North East Atlantic Ocean.
Description
A thick, moderately heavy bivalve shell featuring a smooth, convex exterior dome with a distinct beak (umbo). The outer surface is marked by light radial streaks of warm amber-brown atop a creamy tan base, along with concentric growth lines. Small dark pinholes scattered across the surface indicate post-mortem activity by clionaid sponges.
Key Features
Sub-triangular, smooth convex shape; light brown radial ray coloration; smooth margins; presence of shallow concentric growth rings; prominent central umbo.
Collector Value
Low monetary value (under $5 USD). Highly accessible and common, primarily appreciated as an attractive beachcombing souvenir rather than a high-end investment specimen.
Condition Notes
Fair condition. Exterior exhibits slight erosion and numerous tiny sponge-boring holes (Cliona spp.). The shell margin shows minor chipping, and the glossy original luster is largely weathered away. Collectibility Grade: Fair.
Interesting Facts
The tiny pinholes visible across the shell surface are created by predatory or bioeroding sponges (Cliona species) that bore into calcium carbonate shells after the animal dies. Living specimens use a strong, muscular foot to rapidly burrow into sand when threatened by waves or predators.
Ecological Role
Infaunal filter feeder that helps clarify water by consuming phytoplankton and organic detritus. Serves as a vital food source for sea stars, crabs, and bottom-feeding fish like flatfish.
Similar Species
Glycymeris glycymeris (Dog Cockle) - distinguished by a more perfectly circular outline and distinct taxodont hinge teeth. Spisula solida (Thick Trough Shell) - usually paler with stronger concentric ridges.
Beach Finding Tips
Best found after coastal storms or during low spring tides along wide, sandy Atlantic or Mediterranean beaches. Look in the drift line where ocean currents wash up heavier bivalve valves.