Flat-ribbed Ark or Noah's Ark Shell (Australian variation)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Arca, Species: Arca navicularis · Arcidae (Ark clams family) · Bivalve (two-shelled). This specimen is a single valve with an elongated, trapezoidal/boat-like shape.
Species
Arca navicularis
Shell Type
Bivalve (two-shelled). This specimen is a single valve with an elongated, trapezoidal/boat-like shape.
Family
Arcidae (Ark clams family)
Size
Approximately 25-35mm in length. This is an average juvenile/sub-adult size; adults can reach up to 60-70mm.
Color & Pattern
Base color is off-white to cream with distinct reddish-brown zig-zag or zebra-like banding stripes. A significant portion is covered by a dark, fibrous, hairy periostracum (organic coating), which is typical for this genus and helps it blend into rocky substrates.
Rarity
Common. These are frequently found on beaches near rocky outcrops or mudflats in the Brisbane region.
Habitat
Intertidal and shallow subtidal zones. They are typically found attached to rocks, reef platforms, or dead coral by a strong bundle of threads called a byssus.
Geographic Range
Indo-West Pacific region, very common along the Queensland coast, including Moreton Bay (Brighton/Sandgate areas).
Description
An elongated, sturdy bivalve shell with a characteristic 'boat' shape. The umbo (beak) is situated towards the anterior end, and the hinge line is remarkably straight. The exterior features fine radial ribs intersected by growth lines, creating a slightly decussated texture. The most striking visual element is the contrast between the white/brown striped shell and the dark, shaggy organic coating.
Key Features
Straight hinge line with numerous small teeth (taxodont dentition), elongated trapezoidal shape, and a distinct 'byssal notch' on the ventral margin where the foot/threads emerge.
Collector Value
Low. While beautiful, they are a common species. Value lies mainly in educational or decorative sets rather than high-end investment collecting ($1-$5 USD).
Condition Notes
Fair to Good. The shell retains much of its original periostracum, which is excellent for biological study but obscures the pattern. There is some minor wear on the anterior edge and slight bleaching of the stripes.
Interesting Facts
The name 'Ark' comes from the resemblance of the two shells joined together to a wooden boat or Noah's Ark. Unlike many clams that burrow in sand, Arks use a 'byssus' to bolt themselves to hard surfaces, much like mussels.
Ecological Role
Suspension feeders that filter plankton from the water column. They provide micro-habitats for other small encrusting organisms on their shaggy shells. Currently not listed as a conservation concern.
Similar Species
Arca noae (Mediterranean Ark) and Arca ventricosa. Distinguishable by the local Australian range and the specific ribbing density of A. navicularis.
Beach Finding Tips
Look in the 'wrack line' (debris line) after high tide, especially in areas with rocky mudflats like those found between Sandgate and Brighton. Look for dark, fuzzy-looking 'stones' as the periostracum often camouflages the shell.
Notes
Brighton and Sandgate beach Brisbane