Striate Ark Shell (often referred to locally as the Boat Shell or Ark)
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Tegillarca, Species: Tegillarca granosa · Arcidae (Ark clams/Ark shells) · Bivalve (fragment). This specimen is a damaged valve (half shell) with a trapezoidal/oval shape typical of the family.
Species
Tegillarca granosa (Confidently identified as an Ark Shell, likely T. granosa or Barbatia sp. fragment)
Shell Type
Bivalve (fragment). This specimen is a damaged valve (half shell) with a trapezoidal/oval shape typical of the family.
Family
Arcidae (Ark clams/Ark shells)
Size
Approximately 25-30mm in length. This is a small specimen or a fragment of a larger adult; adults typically reach 50-80mm.
Color & Pattern
Pale chalky white to greyish-green base; the greenish tint suggests algal staining or presence of a weathered periostracum. A distinct dark brown/black ligament or muscle scar remnant is visible near the hinge area.
Rarity
Very Common. These shells are among the most abundant bivalve remains found on the mudflats and beaches of Moreton Bay.
Habitat
Found in intertidal and shallow subtidal zones, particularly in muddy or sandy substrates of estuaries and sheltered bays like Moreton Bay.
Geographic Range
Indo-West Pacific region; extremely common along the Queensland coast, including the Brisbane area (Brighton/Sandgate).
Description
A sturdy, thick-walled bivalve fragment. It features strong radial ribs that are characteristic of the Ark family. The shell is heavily weathered (beach-worn), with the natural white color obscured by environmental staining. The hinge line, though damaged, shows the taxodont dentition (a row of small teeth) typical of Arcidae.
Key Features
Strong, prominent radial ribs; boxy or boat-shaped profile; taxodont hinge (multiple small teeth in a row); and the dark stained ligament area.
Collector Value
Minimal. Due to the high abundance and the damaged/weathered state of this specimen, it holds little monetary value, though it serves as a great example of local estuarine fauna.
Condition Notes
Poor to Fair. The shell is a fragment with significant chipping and erosion around the margins. It is beach-worn and has lost its original white luster and fibrous brown periostracum. Collectibility grade: Poor (Geological/Study specimen).
Interesting Facts
Ark shells are unique because they have red blood containing hemoglobin, unlike most mollusks which have hemocyanin (blue blood). This allows them to survive in the low-oxygen environments of muddy estuaries like Sandgate.
Ecological Role
Suspension feeders that filter organic particles from the water. They are a primary food source for many shorebirds and marine predators like mud crabs.
Similar Species
Anadara trapezia (Sydney Cockle) - distinguished by a more inflated, heart-shaped profile when both valves are together, and thicker ribs.
Beach Finding Tips
Look along the high tide line at Sandgate or Brighton during low tide. They are often found half-buried in the mud or amongst the debris of mangroves.
Notes
Brighton/Sandgate beach Brisbane