Edible oysters belong to two different genera. The Ostrea and Crassostrea differ in appearance but have similar tastes. While the Ostrea have round and scalloped shells, the Crassostrea have shells that are long and asymmetrical. Crassostrea virginica and Crassostrea gigas are Atlantic and Pacific oyster, respectively. Ostrea lurida is found in the pacific while Ostrea edulis is found in the coastal waters of Great Britain. Oysters are bivalves, with two roughly equal hard fluted shells encasing a soft inner body. Generally oysters have a creamy texture and a salty taste, but the state of oysters can differ based on the temperature of the water in which they live. Generally colder water species have superior raw tastes, but it is generally accepted that once cooked the taste difference is not noticeable. The salinity of the water, its mineral content and the chlorophyll content of the plankton in the water and all affect the taste of the oysters. Cooking must be achieved slowly or the texture of the body turn tough.
Oysters have the highest zinc content of any food at roughly 35 mg per 100 gram serving. The same 100 gram serving would also contain roughly 550 mg of the polyunsaturated fatty acid eicosapentaenoic acid and a similar amount of the polyunsaturated fatty acid docosahexaenoic acid. The fat within oyster is therefore highly unsaturated omega-3 fat, although there is around 105 grams of cholesterol per 100 grams. Vitamin B12 is also present in oysters in reasonable quantities as are the minerals copper, iron and selenium. However, as with all organic life, the mineral content of the tissues depends to a large extent on the mineral composition of the environment in which the organism lives. The historical use of oysters as an aphrodisiac likely relates to the zinc content, zinc being required for proper male reproductive function. Because oysters spoil quickly, it is best to only eat them quickly. Oysters have a superior taste in the autumn winter and early spring before they spawn and become less tender.