Caribbean Ark (also known as the Incongruous Ark)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Anadara, Species: A. brasiliana · Arcidae (Ark clams) · Bivalve (Single valve fragment, likely a right valve). Shape: Subquadrate and inflated.
Species
Anadara brasiliana
Shell Type
Bivalve (Single valve fragment, likely a right valve). Shape: Subquadrate and inflated.
Family
Arcidae (Ark clams)
Size
Approximately 0.75 inches (20mm). This is a juvenile specimen; adults typically reach 1.5 to 3 inches.
Color & Pattern
Uniformly chalky white. The lack of brown periostracum (fuzzy outer coating) and clean white color suggests it has been sun-bleached and tumbled by surf.
Rarity
Very Common. These are among the most frequently found shells on Caribbean beaches.
Habitat
Shallow water marine environments, typically found in sandy or muddy substrates in the intertidal and subtidal zones.
Geographic Range
Western Atlantic from North Carolina to Brazil, very common throughout the Caribbean and Gulf of Mexico, including the Cancun area.
Description
A sturdy, thick-walled bivalve shell with a boxy, inflated appearance. It features prominent radial ribs that are crossed by fine concentric growth lines, giving the ribs a slightly beaded or 'toothed' texture near the margins.
Key Features
Heavy shell with 26-28 strong radial ribs; the valves are asymmetrical (incongruous), with one valve overlapping the other at the margin; taxodont hinge (a straight row of many small teeth).
Collector Value
Low monetary value ($0.50-$1.00) due to high abundance. However, it is an essential 'staple' shell for teaching coastal biology or for use in hobbyist crafts.
Condition Notes
Fair to Good. It is a single valve rather than a paired set. The shell shows some minor erosion on the ridges and is sun-bleached, but the overall structure and ribbing are well-defined.
Interesting Facts
Ark clams are unique because they have red blood containing hemoglobin, unlike most mollusks which use hemocyanin. This specimen is called an 'Ark' because the paired valves resemble the hull of an ancient ship (Noah's Ark).
Ecological Role
Suspension feeder that filters plankton and organic matter from the water column. They serve as a food source for gastropods like whelks and for shorebirds.
Similar Species
Anadara floridana (Blood Ark), which is larger and more elongated, and Scapharca transversa (Transverse Ark), which is less boxy in shape.
Beach Finding Tips
In Cancun, look along the drift line after a high tide or storm. They often wash up in large numbers and are easily spotted against darker sands due to their white color.
Notes
Cancun