True oysters are members of the family Ostreidae. This family includes the edible oysters, which mainly belong to the genera Ostrea, Crassostrea, Ostreola and Saccostrea. Examples include the Belon oyster, Eastern oyster, Olympia oyster, Pacific oyster, Sydney rock oyster and the Wellfleet oyster.
Removing a pearl from an oyster.
Almost all shell-bearing molluscs can secrete pearls, yet most are not very valuable.
Pearl oysters are not closely related to true oysters, being members of a distinct family, the feathered oysters (Pteriidae). Both cultured pearls and natural pearls can be obtained from pearl oysters, though other molluscs, such as the freshwater mussels, also yield pearls of commercial value.
The largest pearl-bearing oyster is the marine Pinctada maxima, which is roughly the size of a dinner plate.
Not all individual oysters produce pearls naturally. In fact, in a harvest of three tons of oysters, only three to four oysters produce perfect pearls. Western Australia is currently the worlds largest cultivator of Pinctada maxima oysters.
In nature, pearl oysters produce natural pearls by covering a minute invading parasite with nacre, not by ingesting a grain of sand. Over the years, the irritating object is covered with enough layers of nacre to form what is known as a pearl. There are many different types, colours and shapes of pearl; these qualities depend on the natural pigment of the nacre, and the shape of the original irritant.
Pearl farmers can culture a pearl by placing a nucleus, usually a piece of polished mussel shell, inside the oyster. In three to six years, the oyster can produce a perfect pearl. These pearls are not as valuable as natural pearls, but look exactly the same. In fact, since the beginning of the 20th century, when several researchers discovered how to produce artificial pearls, the cultured pearl market has far outgrown the natural pearl market. Natural pearls have become increasingly scarce, and a necklace with only natural pearls can easily cost several hundred thousand US dollars.
Fishing from the wild
Oysters are harvested by simply gathering them from their beds. In very shallow waters they can be gathered by hand or with small rakes. In somewhat deeper water, long-handled rakes or oyster tongs are used to reach the beds. Patent tongs can be lowered on a line to reach beds that are too deep to reach directly. In all cases the task is the same: the oysterman scrapes oysters into a pile, and then scoops them up with the rake or tongs.
In some areas a scallop dredge is used. This is a toothed bar attached to a chain bag. The dredge is towed through an oyster bed by a boat, picking up the oysters in its path. While dredges collect oysters more quickly, they heavily damage the beds, and their use is highly restricted. Until 1965 Maryland limited dredging to sailboats, and even since that date motor boats can be used only on certain days of the week. These regulations prompted the development of specialized sailboats (the bugeye and later the skipjack) for dredging.
Oysters can also be collected by divers.
In any case, when the oysters are collected, they are sorted to eliminate dead animals, bycatch (unwanted catch), and debris. Then they are taken to market where they are either canned or sold live.
found on life.com
found on wikipedia.org