Flat oysters are bivalve molluscs belonging to the family Ostreidae. They are roughly egg-shaped with valves of unequal size and shape. The shell margin is slightly irregular and is white or cream in colour on the inner surface and purplish-green to olive-brown or gray on the outer shell surface. They are plankton feeders, filtering algae from the water.
The flat oyster can attain around 25 cm in diameter, but are commonly less than 10 cm.
The flat or native oyster is found throughout the marine and estuarine habitats of southern Australia, from mid NSW to the southern parts of Western Australia and Tasmania. They usually attach themselves to hard substrates before later breaking free to settle on sand or soft mud. These oysters live in the subtidal zone from about 2 to 20 m depth.
The environment in which an oyster lives can largely determine its shape (ie. they can be irregular in shape when attached to hard rocky bottoms or very flat when found in soft bottoms). This sometimes makes it difficult to identify oyster species. To distinguish and classify oyster species it is often useful to examine the internal characteristics.