The Angasi oyster (Ostrea angasi), also called the southern mud oyster or flat oyster, is a large, rounded oyster with a cup-shaped lower valve and a flat or slightly concave upper valve that fits within. Juveniles grow attached to stones or shells, but older animals live free in soft sediment. The Angasi is a native Australian oyster that was once plentiful around southern Australian shores from New South Wales to Fremantle, including Tasmania and was an important part of the diet for all indigenous Australians living on or near the coast. The shell remains a common component of many coastal aboriginal middens.
Oyster farming is the oldest aquaculture industry in Australia, commencing in the late 1800s. The local natural oyster beds were nearly all exhausted by the turn of the 19th century and have not recovered. The populations crashed because most of the oysters were torn out of the estuaries by dredging to be used to make lime, critical for the building industry. In turn, the Sydney rock oyster was seen as a better farming proposition and it replaced the Angasi in oyster farms. It is also likely that a parasitic protozoan has contributed to the decline of the Angasi.
Today, Angasi oysters represent between 0.03 and 0.04% of all Australian oyster production. They have attained the status of a food delicacy among informed consumers and are highly regarded due to their flavour. However, oysters around the world are endangered as a result of increased pollution and overuse of waterways. The Angasi oyster is more difficult to cultivate as the spat (young oysters) have thinner shells and they require gentler handling and have a shorter shelf life than other oysters. Two New South Wales farms have been closed due to Pacific Oyster Mortality Syndrome. Given the increase in global shipping with associated inappropriate discharge of ballast, the Angasi is under increasing threat to its existence.