Ostrea edulis

When mature, O. edulis adults range from 3.8 to 11 centimetres (1.5 to 4.3 in) across.[5]

Shells are oval or pear shaped, white, yellowish or cream in colour, with a rough surface showing pale brown or bluish concentric bands on the right valve. The two valves are quite different in shape and size, as the left one is concave and fixed to the substratum, while the right one is almost flat and fits inside the left. The inner surface is smooth, whitish or bluish-grey.[6]

Fossil of Ostrea edulis from Pliocene of Italy

Ostrea edulis are gregarious molluscs that start their lives as males. They mature sexually after eight–ten months and may change sex depending on the water temperature. Usually the lifespan can reach about six years, with a maximum of 15 years. Adult oysters feed by filtration.[7]

Ostrea edulis; a) labial palpi b) gills c) mantle d) junction of the two folds of the mantle e) large adductor muscle f) the shell

The species naturally ranges along the western and southern coasts of Europe from Norway to Morocco and including most of the British Isles and the Mediterranean coast.[8] Naturally viable populations have appeared in eastern North America from Maine to Rhode Island subsequent to artificial introduction in the 1940s and 1950s.[8]

Ostrea edulis can be found in estuarine and shallow coastal water with hard substrata of mud and rocks.[9]

Worldwide O. edulis harvest in tonnes, 1950–2003

Ostrea edulis has been harvested throughout Europe as an important food source since prehistory.[10] During Roman occupation of Britain O. edulis oysters were exported in large quantities back to Italy.[11] However, due to their robust nature and ease of cultivation the Pacific oysters, Crassostrea gigas, account for more than 75 percent of Europe's oyster production.

European flat oysters are famously grown in Brittany, France. The true Belon oyster is cultivated in the Belon River, France, and has the AOC protected name.[12] In the 1950s, Dutch scientists artificially introduced Belon oyster seed into the waters around Maine in hopes to establish a viable stock. The initial project was abandoned but ten years later natural colonies of flat oysters were found in the wild.[3] Many North American suppliers use the name 'Belon' to species that are found in the wild throughout the United States.

Loch Ryan oysters are sourced from Scotland's only commercial European flat oyster bed.

Ostrea edulis is now also being maricultured in the states of California, Maine, and Washington in the United States. The species once dominated European oyster production but disease, pollution, and overfishing sharply reduced the harvest.[8]

U.S. oyster growers farm O. edulis in small quantities on both coasts. They are prized for their unique tannic seawater flavour, sometimes described as dry and metallic, and are more expensive than other American oysters.[8] The flavour is considered excellent for eating raw on the half shell.[13][14]

The abductor muscle of the European flat in combination with the shape of the shell results in a somewhat weaker seal compared with other oyster species. It is common practice to use rubber bands to prevent oysters from spilling their liquor and dehydrating in storage before consumption.[15]

• Locmariaquer#History of oyster farming

Wikimedia Commons has media related to Ostrea edulis.

• UK Biodiversity Action Plan