Incongruous Ark (alternately called the Blood Ark or Little White Ark)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Anadara, Species: A. brasiliana · Arcidae (Ark clams family) · Bivalve (single valve shown); Rhomboidal/Boxy shape
Species
Anadara brasiliana
Shell Type
Bivalve (single valve shown); Rhomboidal/Boxy shape
Family
Arcidae (Ark clams family)
Size
Approximately 1 to 1.5 inches in length. This is a juvenile or small adult specimen; adults typically reach 2-3 inches.
Color & Pattern
Uniform creamy white to dull tan. This specimen appears weathered or sun-bleached, losing the dark, hairy periostracum (outer skin) that usually covers live individuals.
Rarity
Very Common; these are frequently found on beaches across the southeastern United States and the Caribbean.
Habitat
Found in shallow marine waters and intertidal zones, typically buried in sandy or muddy substrates in warm temperate and tropical seas.
Geographic Range
Western Atlantic, ranging from North Carolina down through the Caribbean to Brazil and the Gulf of Mexico.
Description
A sturdy, thick-walled bivalve characterized by deep radial ribs. The shell has a distinctive box-like or rhomboidal shape with a high, prominent umbo (beak). The ribs are often beaded or scaly in fresh specimens, though this one shows signs of beach-rolling wear.
Key Features
Heavy radial ribbing (approximately 26-30 ribs), a straight hinge line with many small teeth (taxodont dentition), and left/right valves that are slightly unequal in size (incongruous).
Collector Value
Minimal financial value (typically less than $1.00 USD). It is a staple specimen for beginners or for use in crafts due to its abundance and durability.
Condition Notes
Fair condition. The valve shows significant erosion and rounding of the ribs, suggesting it was tumbled in the surf for some time. The lack of a periostracum and slightly dull luster indicates a beach-worn find rather than a live-collected shell.
Interesting Facts
The common name 'Blood Ark' refers to the fact that these clams have hemoglobin in their blood, which is rare for mollusks and gives their soft tissue a bright red appearance.
Ecological Role
Suspension feeders that filter plankton from the water column. They serve as a food source for gastropods like whelks and for shorebirds.
Similar Species
Ponderous Ark (Noetia ponderosa), which is larger and more heart-shaped from a side profile; Blood Ark (Anadara ovalis), which is more rounded and less boxy.
Beach Finding Tips
Look along the high-tide line after a storm. They are very heavy for their size, so they often settle in 'shell pockets' with other weighted debris like stones and thick bivalve fragments.