Transverse Ark (Alternate common names: Blood Ark, Cut-ribbed Ark)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Anadara, Species: A. transversa · Arcidae (Ark shell family) · Bivalve, whole shell, oblong/trapezoidal shape
Species
Anadara transversa
Shell Type
Bivalve, whole shell, oblong/trapezoidal shape
Family
Arcidae (Ark shell family)
Size
Approximately 1.5 inches in length. This is an average to slightly larger than average size for this species, which typically ranges from 1 to 2 inches.
Color & Pattern
Uniform grayish-blue due to weathering and fossilization/beach staining, with fine radial ribs. The interior is typically whitish, though not fully visible here.
Rarity
Common
Habitat
Inhabit shallow estuarine and marine waters, living partially buried in muddy or sandy substrates from the intertidal zone down to shallow subtidal depths.
Geographic Range
Western Atlantic Ocean, ranging from Massachusetts south to Florida, the Gulf of Mexico, the Caribbean, and occasionally down to Brazil.
Description
A sturdy, elongated bivalve shell featuring prominent radial ribs radiating outward from the beaks. The hinge line is straight with numerous small, interlocking teeth typical of ark shells, and the posterior end exhibits a characteristic angular slope.
Key Features
Elongated oblong or trapezoidal outline, straight hinge line set with numerous small taxodont teeth, and distinct, numerous radial ribs crossed by fine concentric growth lines.
Collector Value
Low to moderate value. While very common and not typically valuable as an individual collector piece, it is a quintessential find for regional beachcombers along the Atlantic coast.
Condition Notes
Weathered and worn condition with a dull gray, beach-worn appearance. The surface shows minor erosion but the overall valve structure is intact. Collectibility grade is Good.
Interesting Facts
Unlike most other bivalves, ark shells like the Transverse Ark possess a red-colored blood containing hemoglobin, which allows them to survive in low-oxygen environments.
Ecological Role
Functions as a filter feeder, helping to clear water columns of phytoplankton and particulate matter. They serve as an important prey item for predatory gastropods, fish, and shorebirds.
Similar Species
Bloody Cockle (Anadara ovalis) and Ponderous Ark (Noetia ponderosa). The Transverse Ark is distinguished by its more elongated, trapezoidal shape and sharper posterior angle compared to the more rounded Ovalis.
Beach Finding Tips
Look along high-tide wrack lines on ocean-facing beaches and sound-side shores, especially after storms or dredging operations that churn up offshore benthic sediments.
Notes
Outer banks, North Carolina on Kill Devils Hill beach