<P> Oysters have been cultured for well over a century . The Pacific oyster (Magallana gigas) is presently the most widely grown bivalve around the world . Two methods are commonly used, release and bagging . In both cases, oysters are cultivated onshore to the size of spat, when they can attach themselves to a substrate . They may be allowed to mature further to form' seed oysters' . In either case, they are then placed in the water to mature . The release technique involves distributing the spat throughout existing oyster beds, allowing them to mature naturally to be collected like wild oysters . Bagging has the cultivator putting spat in racks or bags and keeping them above the bottom . Harvesting involves simply lifting the bags or rack to the surface and removing the mature oysters . The latter method prevents losses to some predators, but is more expensive . </P> <P> The Pacific oyster has been grown in the outflow of mariculture ponds . When fish or prawns are grown in ponds, it takes typically 10 kg (22 lb) of feed to produce 1 kg (2.2 lb) of product (dry - dry basis). The other 9 kg (20 lb) goes into the pond and after mineralization, provides food for phytoplankton, which in turn feeds the oyster . </P> <P> To prevent spawning, sterile oysters are now cultured by crossbreeding tetraploid and diploid oysters . The resulting triploid oyster cannot propagate, which prevents introduced oysters from spreading into unwanted habitats . </P> <P> In many areas, non-native oysters have been introduced in attempts to prop up failing harvests of native varieties . For example, the eastern oyster (Crassostrea virginica) was introduced to California waters in 1875, while the Pacific oyster was introduced there in 1929 . Proposals for further such introductions remain controversial . </P>

How many types of oysters are there in the world