As the name indicates, the European flat oyster is flat and usually has a greyish shell with a wavy, slightly flaky structure. It is a thermophilic species that needs a water temperature of more than 16–18°C to become sexually mature. This means that it is found in areas where the water becomes warm enough in the summer, for example, in protected coves. The European flat oyster can switch between being female and male, and the sex change is dependent on the temperature. It lives attached to the substrate, usually on stones or mussel shells from just below the low tide mark to a depth of two metres. It is common in the Black Sea, the Mediterranean Sea, along the coast of Morocco and northwards along the European coasts north to Trøndelag
The molluscs' diet consists of phytoplankton, micro-organisms, bacteria and dead organic material, and the phytoplankton is the most important nourishment. It is the water which transports the nourishment to the molluscs, and factors such as depth, tides and water movement affect the access of the molluscs to food. These factors, together with the production of phytoplankton, mean that both the quantity and quality of oysters vary.
European flat oysters spawn in the summer, when the water temperature is high. The fertilisation itself occurs inside the female's mantle cavity where the eggs are fertilised, and the larvae develop inside the mother for the first week. When they leave the mother, they live for one to two weeks in open waters before they migrate to the bottom and attach themselves.
Farming of flat oysters occurs in two phases, spat and edible oyster production. Traditionally the oyster spat are produced in protected coves heated by the sun. The brood stock oysters spawn freely in the water column and the spat are collected on various types of collectors. Today spat are also produced in hatcheries or by a combination of hatchery and constricted inlet. Then the oysters are further cultured in tray systems in the sea or in protected coves until they reach a marketable size. European flat oysters are picked by hand, and they can be harvested year round.
Commercial harvesting of European flat oysters is done today by a small number of operators. The species is also harvested for private use.
Commercial farming of oysters is primarily regulated by the Norwegian Aquaculture Act. The Act specifies that aquaculture is to be established, conducted and wound up in an environmentally-responsible manner. Aquaculture is supervised by a number of authorities, such as the Norwegian Food Safety Authority, the Norwegian Directorate of Fisheries, the Norwegian Coastal Administration, the Norwegian Water Resources and Energy Directorate and the County Governor.
Food safety/quality control
The Norwegian seafood industry is subject to stringent requirements in order to ensure food safety. The control system consists of several bodies which jointly examine and monitor compliance with the requirements in all stages of the production chain. The bodies which supervise food are the Norwegian Food Safety Authority, the Norwegian National Institute of Nutrition and Seafood Research (NIFES), the Norwegian Directorate of Fisheries and the Norwegian Ministry of Trade, Industry and Fisheries.
Bivalves which are harvested for sale must be safe to eat. Commercial farming of molluscs may only be done in areas classified by the Norwegian Food Safety Authority. Molluscs which are harvested for direct consumption must be harvested from class A areas. If harvesting is done from B or C areas, the molluscs must be cleaned or heat-treated before sale.
The quality is best from autumn and through the winter when the molluscs have good access to nourishment, so that they have gathered up a great deal of stored food. The quality can be good in the spring, but sexual maturation and spawning reduce it somewhat.
European flat oysters are rich in:
- Protein, which builds and maintains all the cells in the body.
- Vitamin D, which is important for the body's proper calcium balance, which maintains and strengthens the bones in the body.
- Vitamin B12, which is important for the body being able to produce new cells, including red blood cells, and can contribute to preventing anaemia.
- Selenium, an important element in the enzymes that combat harmful chemical processes in the body.
- Iodine, which is important for the body's production of hormones which play a role in regulating metabolism.
European flat oysters are a sought-after ingredient with their fresh taste of algae, mixed with a unique sweet, mineral aftertaste. The entire shell meat of European flat oysters is served, and it is very sweet, rich and tasty. European flat oysters can be served raw or heat-treated, and a little lemon and pepper is usually added. They are often served as an appetiser.