The southern mud oyster, Australian flat oyster, native flat oyster, native mud oyster, or angasi oyster (Ostrea angasi), is endemic to southern Australia, ranging from Western Australia to southeast New South Wales and around Tasmania. Ostrea angasi superficially resembles Ostrea edulis and both species may be referred to with the name "flat oyster". However, the two species do not occur naturally in the same geographic distribution.

Habitat

This species is found in sheltered, silty or sand-bottomed estuaries at depths between 1 and 30 metres.

Diet

Flat oysters, like all other oyster species, are filter feeders, feeding on, and taking in anything small enough to be filtered in their gills. This may include plankton, microalgae or inorganic material.

Predators

Oyster growers at Coffin Bay, South Australia have observed stingrays eating their experimental commercial stocks of Ostrea angasi.

Commercial harvesting

Extensive oyster reefs in southern Australia were largely destroyed by over-exploitation during the 19th and early 20th Centuries. Oysters were dredged directly from the seabed. Once common, Ostrea angasi became locally extinct, in oyster-producing estuaries on the East Coast north of the Clyde River, as a result of the accidental introduction of the mud worm, Polydora websteri, from New Zealand, in 1888 to 1898. The mudworm and silting ended all sub-tidal oyster production in New South Wales and Southern Queensland, and the oyster industry there became totally dependent upon inter-tidal production of another indigenous oyster, the Sydney rock oyster, Saccostrea glomerata. In the 21st century, commercial oyster growers in southern Australia have started experimentally farming O. angasi as a means to diversify their businesses. This was prompted by other growers suffering massive stock losses of Crassostrea gigas resulting from outbreaks of Pacific Oyster Mortality Syndrome (POMS).

Not-for-profit projects

Port River

The not-for-profit organisation Estuary Care Foundation was established in South Australia to undertake trials growing Ostrea angasi in the Port River and adjacent waters. The organisation is also involved in seagrass monitoring and restoration work within the Port River.

Windara Reef

Windara Reef was constructed in Gulf St Vincent, offshore from Ardrossan, to promote the reestablishment of Ostrea angasi. The reef was also opened to recreational fishers in 2017. As of April 2019, it was the largest shellfish reef restoration project in the southern hemisphere. The Nature Conservancy, the Australian Government, the South Australian Government, the Yorke Peninsula Council, The University of Adelaide and the Ian Potter Foundation have each contributed to funding the project.

Georges River and Botany Bay

It was announced, in 2023, that work would begin on re-establishing three sub-tidal oyster reefs in the Georges River estuary in New South Wales (at Audrey Bay, Coronation Bay and on the eastern side of Taren Point), and another sub-tidal reef downstream in Botany Bay. The reefs will support reintroduced Ostrea angasi—locally extinct in those waters since 1896—and also the now depleted Sydney Rock Oyster. The oyster reefs are expected to increase both water quality and biodiversity, and increase the numbers of fish.