Transverse Ark (Alternate names: Transverse Ark Clam)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Anadara, Species: A. transversa · Arcidae (Ark shell family) · Bivalve (two-shelled), whole shell, elongated oval shape
Species
Anadara transversa
Shell Type
Bivalve (two-shelled), whole shell, elongated oval shape
Family
Arcidae (Ark shell family)
Size
Approximately 1 to 1.5 inches in length, which is average for this species. Adults typically range from 0.75 to 1.5 inches.
Color & Pattern
Off-white to grayish-white base color with faint brownish tinges along the margins and growth lines. The surface lacks bright colors, typical of many burrowing ark clams, showing natural weathering.
Rarity
Common
Habitat
Found in shallow marine and estuarine waters, typically from the intertidal zone down to moderate depths of several fathoms. Prefers muddy or sandy substrates and seagrass beds.
Geographic Range
Western Atlantic Ocean, ranging from Massachusetts south through the Gulf of Mexico, the Caribbean, and down to South America.
Description
An asymmetrical, elongated bivalve featuring strong, coarse radial ribs intersecting with fine concentric growth lines. The hinge line is straight with numerous small, interlocking teeth typical of ark shells, and the overall form is oblong.
Key Features
Elongated, trapezoidal to oblong outline; prominent radial ribs (usually around 30 to 35); straight, elongated hinge line with a series of transverse teeth; inequilateral valves.
Collector Value
Low to moderate commercial value. While widely collected by beachcombers, they are very common and typically valued as educational or crafting specimens rather than high-end collector pieces.
Condition Notes
Good condition. The single valve shown is intact with clear sculptural ribs and minimal edge wear, grading as a 'Good' to 'Fine' beach specimen.
Interesting Facts
Unlike many bivalves that burrow completely out of sight, ark shells possess simple eyes along their mantle edges that can detect changes in light and shadow. They also have specialized red blood containing hemoglobin, which is unusual for mollusks.
Ecological Role
As filter feeders, ark shells help maintain water clarity by consuming phytoplankton and organic particulates. They serve as an important food source for marine predators such as rays, large gastropods, and bottom-feeding fish.
Similar Species
Blood Ark (Anadara ovalis) and Ponderous Ark (Noetia ponderosa). The Transverse Ark is distinguished by its more elongated, oblong shape and finer, numerous radial ribs compared to the rounder, heavier species.
Beach Finding Tips
Look along sandy and muddy estuarine beaches after storms or high tides. Searching near salt marshes and tidal inlets often yields washed-up single valves.