Incongruous Ark (also known as the Blood Ark or Blood Clam)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Anadara, Species: A. brasiliana · Arcidae (Ark shells) · Bivalve (Two-shelled); specifically a single valve with an inflated, box-like trapezoidal shape.
Species
Anadara brasiliana (synonym: Scapharca brasiliana)
Shell Type
Bivalve (Two-shelled); specifically a single valve with an inflated, box-like trapezoidal shape.
Family
Arcidae (Ark shells)
Size
Approximately 1 inch (25mm) in length. This is a juvenile to medium specimen; adults can reach 2 to 3 inches.
Color & Pattern
Overall creamy white to pale tan base with notable orange-tinted bands following the growth lines. The internal surface (hidden) is typically white. The orange hue often suggests the presence of iron minerals or natural pigment common in West Atlantic Arks.
Rarity
Very Common. This is one of the most frequently encountered shells on beaches in the Southeastern United States and Gulf Coast.
Habitat
Inhabits shallow marine waters, typically in sandy or muddy substrates. They are found from the intertidal zone down to depths of about 50 meters.
Geographic Range
Western Atlantic region, ranging from North Carolina down to Brazil, and very common throughout the Gulf of Mexico and Caribbean.
Description
A sturdy, thick-walled bivalve characterized by its roughly trapezoidal shape and prominent radial ribs. The shell is remarkably 'inflated' or puffy. It features around 26 to 28 square-edged radial ribs which, in this specimen, show clear horizontal growth lines crossing them.
Key Features
The defining feature is the 'incongruous' overlap—the left valve is slightly larger and overlaps the right valve at the margin. It also features a straight hinge line with numerous small teeth (taxodont dentition).
Collector Value
Low. Because they are ubiquitous on many beaches, they have little monetary value but are excellent educational specimens for beginning collectors.
Condition Notes
Fair to Good. The shell shows some signs of beach wear (rounded edges on the ribs) and lacks the dark brown, hairy periostracum (outer skin) usually found on live specimens, indicating it is a beach-worn find.
Interesting Facts
The ‘Blood Ark’ nickname comes from the fact that these mollusks produce hemoglobin in their blood to help them survive in low-oxygen environments, causing their soft tissues to appear bright red, which is unusual for a mollusk.
Ecological Role
A filter feeder that helps clear particulates from the water column. They are a significant food source for gastropods like whelks and for shorebirds.
Similar Species
Ponderous Ark (Noetia ponderosa), which is larger and more heart-shaped when both valves are joined; Blood Ark (Anadara ovalis), which is more rounded and has fewer ribs.
Beach Finding Tips
Look for them in the 'shell hash' piles at the high tide line after a storm. They are very heavy for their size and often settle in areas where heavier debris is deposited.