Saddle Oyster (also known as the Jingle Shell and Cap Shell)
Phylum: Mollusca, Class: Bivalvia, Order: Pectinida, Family: Placunidae, Genus: Placuna, Species: placenta · Placunidae (Windowpane Oyster family) or Isognomonidae (Tree Oyster family) · Bivalve (fragmented single valve). Shape is irregularly ovate/sub-rectangular.
Species
Placuna placenta (Windowpane Oyster) or cf. Isognomon ephippium (Saddle Oyster)
Shell Type
Bivalve (fragmented single valve). Shape is irregularly ovate/sub-rectangular.
Family
Placunidae (Windowpane Oyster family) or Isognomonidae (Tree Oyster family)
Size
Estimated at 6-8 cm in length. This is a partial specimen; full adults can reach 15 cm in diameter.
Color & Pattern
Naturally translucent creamy-white to buff base. Notable dark black/brown staining which is likely anaerobic sediment staining (sulfide) from the muddy substrate of Moreton Bay. Nacreous/pearly interior luster is slightly visible.
Rarity
Common. These are frequently found as fragments on Brighton Beach and Sandgate due to the muddy, intertidal nature of the area.
Habitat
Shallow, muddy or silty marine environments. Found in protected bays, estuaries, and mudflats, often partially buried in soft substrate.
Geographic Range
Indo-West Pacific region, ranging from northern Australia (including Brisbane's Moreton Bay) to Southeast Asia and the Philippines.
Description
An irregularly shaped, wafer-thin calcified shell fragment. The shell is characterized by its extreme thinness and translucency. While complete specimens are circular and flat, this beach-worn fragment shows the layered, foliated structure of the shell's crystalline calcium carbonate. It has a smooth but slightly undulating surface with a faint pearly sheen in non-stained areas.
Key Features
Translucency (allows light through even when thick), nacreous (pearly) interior, foliaceous or layered shell structure, and a flat/compressed profile.
Collector Value
Minimal financial value due to fragmented condition. Species is highly common. Its value is primarily educational or for craft use.
Condition Notes
Poor to Fair. This is a beach-worn fragment with significant breakages along the margins. It displays heavy environmental staining (blackened patches) from sulfur-rich mud. Not a 'gem' specimen but an excellent representative of local estuarine taphonomy.
Interesting Facts
In Southeast Asia, these shells were traditionally used as window panes before glass was widely available because they are translucent enough to let light pass through while blocking wind. They are also used in the 'Capiz' shell craft industry for lampshades and ornaments.
Ecological Role
Suspension feeders that filter plankton from the water column. They help stabilize silty bottoms and provide a hard substrate for smaller organisms like bryozoans to attach to. They are sensitive to heavy siltation and pollution.
Similar Species
Anomia ephippium (common Jingle Shell), which is usually smaller and more rounded, and Isognomon ephippium (Saddle Oyster), which is thicker and more purple/brown.
Beach Finding Tips
Search at low tide along the mudline at Sandgate or Brighton Beach. Look for 'shards' that look like dirty glass or fingernails sticking out of the silty sand.
Notes
Brighton beach Sandgate in Brisbane