Transverse Ark (also known as Mossy Ark or Plicated Ark)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Anadara, Species: Anadara transversa · Arcidae (Ark shell family) · Bivalve, whole shell, asymmetrical inequilateral fan/oval shape
Species
Anadara transversa
Shell Type
Bivalve, whole shell, asymmetrical inequilateral fan/oval shape
Family
Arcidae (Ark shell family)
Size
Approximately 0.75 to 1 inch wide. Adults typically range from 0.5 to 1.5 inches, making this a medium-sized specimen for the species.
Color & Pattern
Uniform off-white to creamy beige with no complex color patterns. The surface shows natural sun-bleached tones typical of beach-worn specimens. The interior matches the exterior, displaying a smooth, white porcelain-like finish.
Rarity
Very Common
Habitat
Inshore and shallow subtidal waters, found burrowed or resting partially exposed in muddy, sandy, or shell-gravel substrates, often in bays, estuaries, and sounds.
Geographic Range
Western Atlantic Ocean, ranging from Massachusetts south along the United States coast, through the Gulf of Mexico, the Caribbean, and down to northern South America.
Description
A sturdy, compact bivalve shell characterized by distinct, strong radial ribs radiating outward from the umbo to the ventral margin. The hinge line is straight with numerous tiny, interlocking teeth typical of ark shells. The shell has a somewhat oblong or trapezoidal outline with a rounded margin.
Key Features
Straight hinge line containing a long row of small, uniform taxodont teeth; prominent, coarse radial ribs; and an inflated, solid valve structure with a slightly offset umbo.
Collector Value
Low to moderate commercial value. While extremely popular and accessible for beginner beachcombers and educational collections, advanced collectors typically only seek out pristine specimens with intact periostracum.
Condition Notes
Good condition with minor wear along the margins. The surface is clean and intact, displaying normal beach-worn smoothing without major fractures or boring holes. Collectibility grade is Good.
Interesting Facts
Unlike many bivalves that burrow deeply, ark shells possess a modified foot that allows them to cling tightly to hard surfaces or seaweed using a network of strong fibers called a byssus, though mature individuals often rest unattached on the bottom.
Ecological Role
Acts as an important filter feeder, helping to clarify coastal waters by consuming phytoplankton and organic particulates. Serves as a vital food source for bottom-feeding fish, rays, crabs, and carnivorous sea snails.
Similar Species
Bloody Ark (Anadara ovalis) and Ponderous Ark (Noetia ponderosa). They can be distinguished by counting the radial ribs and examining the shape of the hinge and posterior margins.
Beach Finding Tips
Look along intertidal mudflats, bay beaches, and storm wrack lines after strong onshore winds. Sifting through shell hash and fine gravel beds in protected estuarine areas yields the best results.