Australian Mud Cockle, Blood Cockle, or Oval Cockle
Phylum: Mollusca, Class: Bivalvia, Order: Arcida, Family: Arcidae, Genus: Anadara, Species: Anadara trapezia · Arcidae (Ark shells or Cockles) · Bivalve (Single valve/half shell); Shape: Equivalve, trapezoidal, with pronounced radiating ribs.
Species
Anadara trapezia
Shell Type
Bivalve (Single valve/half shell); Shape: Equivalve, trapezoidal, with pronounced radiating ribs.
Family
Arcidae (Ark shells or Cockles)
Size
Approximately 45-60mm in length. This is an adult specimen; the species typically grows up to 70-80mm.
Color & Pattern
Exterior shows a gradient from a chalky white near the umbo (hinge) to a dark charcoal grey or blackish-brown towards the outer margin. The dark coloration is largely remaining periostracum (organic skin) that is worn away at the peak. Interior is typically porcelain white.
Rarity
Very Common. These are dominant species in muddy estuaries like Sandgate and are frequently washed up after high tides or storms.
Habitat
Intertidal and shallow subtidal zones. They prefer soft substrate like muddy sand, often found buried in mangrove mudflats or seagrass beds (Zostera).
Geographic Range
Common along the eastern coast of Australia, from Queensland (including Brisbane/Moreton Bay) south to Victoria and Tasmania.
Description
A robust, heavy-set bivalve with a distinct trapezoidal or pear-shaped outline. The shell features approximately 20-25 thick, prominent radiating ribs that are crossed by fine concentric growth lines. The hinge line is straight with many small teeth (taxodont dentition).
Key Features
Distinguished by its heavy weight, the 'pitched' or trapezoidal shape, the thick radiating ribs, and the way the valves interlock with a crenulated margin.
Collector Value
Low monetary value ($1-$5) due to abundance, but high educational and regional value. Specimens with both valves joined (a 'paired' shell) are more desirable for displays.
Condition Notes
Good/Beach Grade. The shell shows natural wear on the umbo and some sand abrasion, but the ribs are well-defined. The dark periostracum is partially intact, which is characteristic of fresh beach finds.
Interesting Facts
Unlike most mollusks, Anadara species have red blood containing hemoglobin, which allows them to survive in low-oxygen environments like deep mud. They were a significant food source for Indigenous Australians, and large shell middens consisting mostly of this species are found along the QLD coast.
Ecological Role
Suspension feeder that filters plankton and organic detritus from the water. They are important indicators of estuarine health and provide a food source for rays, whelks, and shorebirds.
Similar Species
Anadara oceanica (larger, more symmetrical) and various Glycymeris species (which are more circular and lack the trapezoidal posterior).
Beach Finding Tips
Best found at low tide along the muddy stretches of Sandgate and Brighton. Look for them near the base of mangrove roots or in the ripple marks of the mudflats after a receding tide.
Notes
Brighton beach Sandgate in Brisbane