Blood Ark (Alternate: Plicate Ark)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Anadara, Species: Anadara ovalis · Arcidae (Ark shell family) · Bivalve (two-shelled), whole shell valve, fan/asymmetrical triangular shape
Species
Anadara ovalis
Shell Type
Bivalve (two-shelled), whole shell valve, fan/asymmetrical triangular shape
Family
Arcidae (Ark shell family)
Size
Approximately 1 to 1.5 inches, which is medium for this species. Adults typically range from 1 to 2 inches.
Color & Pattern
Muted grayish-white base with dark gray to brownish-black staining, likely due to mineral exposure or age in the sediment. The exterior shows classic radial ribs crossed by fine concentric growth lines, giving a checkered appearance near the margins. Colors are somewhat weathered.
Rarity
Common
Habitat
Found in shallow marine waters, bays, and estuaries, typically buried partially or resting on muddy, sandy, or shell-gravel substrates from the intertidal zone down to moderate depths.
Geographic Range
Western Atlantic Ocean, ranging from Massachusetts south through the Gulf of Mexico, the Caribbean, and down to Brazil.
Description
A sturdy, thick-ivalved ark shell featuring prominent, strongly developed radial ribs running from the umbo to the ventral margin. The valve exhibits a rounded, somewhat inflated shape with a distinct beak (umbo). The texture is rough and sculpured with a grid-like pattern where growth lines intersect the ribs.
Key Features
Strongly inflated valve, numerous prominent radial ribs (usually around 26-35), straight hinge line with a taxodont (row of many small, similar teeth) dentition typical of ark shells, and a rounded outline.
Collector Value
Low to moderate. While very common and not typically valuable as a rare collector specimen, ark shells are popular staples in educational collections and beginner beachcomber displays. Usually valued under $1.
Condition Notes
Single valve (half of a bivalve pair). Shows moderate wear and weathering with some discoloration and loss of the original velvety periostracum. Overall condition is Fair to Good for a beach-worn specimen.
Interesting Facts
Unlike most other bivalves, ark shells like the blood ark possess true red hemoglobin dissolved in their blood, giving their flesh a distinctive red color and allowing them to survive in low-oxygen environments.
Ecological Role
As suspension feeders, ark shells filter phytoplankton and organic particles from the water column, helping to clear water and cycle nutrients. They serve as an important food source for bottom-feeding fish, rays, and larger carnivorous gastropods.
Similar Species
Transverse Ark (Anadara transversa), which is more elongate and rectangular, and the Turkey Wing (Venericardia or Arca zebra), which has distinct zebra-like color banding.
Beach Finding Tips
Look along muddy sand flats, bayside beaches, and storm wrack lines after rough weather. Entire valves or single fragments wash ashore regularly in the intertidal zone.