Window-pane Oyster, Hammer Oyster or Finger Oyster
Phylum: Mollusca, Class: Bivalvia, Order: Ostreida, Family: Isognomonidae, Genus: Isognomon, Species: ephippium · Isognomonidae (Tree Oyster or Finger Oyster family) · Bivalve (Single Valve); Flat, irregular, semi-circular to D-shaped shell
Species
Isognomon ephippium (Confidence Level: 90%)
Shell Type
Bivalve (Single Valve); Flat, irregular, semi-circular to D-shaped shell
Family
Isognomonidae (Tree Oyster or Finger Oyster family)
Size
Approximately 45-60mm in height. This is a mid-sized specimen; adults can reach up to 100-120mm.
Color & Pattern
Exterior is typically dark grey to purplish-black. Interior (shown) features a brilliant nacreous (mother-of-pearl) center with a wide, matte dark-purplish to black margin. Iridescent highlights are visible in the nacre.
Rarity
Common (found regularly on beaches in the Moreton Bay area, though often as single valves or broken fragments).
Habitat
Intertidal and shallow subtidal zones, typically found attached to rocks, mangrove roots, or other shells in muddy or sandy environments.
Geographic Range
Indo-West Pacific region, ranging from East Africa to Northern and Eastern Australia (Queensland coast including Brisbane area).
Description
An irregular, compressed bivalve shell with a distinctive nacreous interior. The hinge line is straight and features a series of parallel grooves (ligament pits). The shell is relatively thin and brittle, often becoming flaked or 'leafy' at the edges. This specimen shows a classic D-shape with a high-luster pearly interior.
Key Features
Multi-vincular hinge (multiple parallel ligament pits), thin compressed profile, and the sharply defined contrast between the inner pearly nacre and the dark, non-nacreous outer margin.
Collector Value
Minimal monetary value ($1-$5 USD), but high educational value for local ecosystem study. Valued by collectors for the unique 'comb-like' hinge structure.
Condition Notes
Fair to Good. This is a single valve with significant erosion on the outer margin and some 'pocking.' The internal nacre is well-preserved, showing good iridescence. Grade: Good (Collector/Beachfind).
Interesting Facts
Members of this family are called 'Tree Oysters' because several species live attached to mangrove roots. Unlike the edible rock oyster, these have a very thin body. They are closely related to pearl oysters (Pinctada).
Ecological Role
Filter feeder that helps clear the water column. They provide habitat for secondary settlers like barnacles and bryozoans. They are not currently considered a threatened species.
Similar Species
Isognomon isognomum (Legume Hammer Oyster), which is much more elongated and 'L-shaped' compared to the rounded/D-shaped ephippium.
Beach Finding Tips
Look along the high tide line at Sandgate and Brighton after a storm or strong easterly winds. They are often found amongst debris or attached to dead mangrove wood.
Notes
Inside of a shell Found in Brighton / Sandgate beach in Brisbane