Stark bedroht und schützenswert

16 December 2019

Photo: UHH/CeNak, Stau

The two oyster species compared, on the right the European oyster (Amrum) and on the left the Pacific (France).

They were once widespread, but today we only know their conspecifics. The European oyster, Ostrea edulis, has been considered extinct in Germany and Belgium since the middle of the 20th century. In order to still be able to meet the demand for oysters despite extinction in certain areas, the Pacific oyster, Crassostrea gigas, was finally introduced. This larger species was able to spread widely. Living we know only the Pacific oyster from the North Sea, but the shells of our treasure of the month can still be discovered today.

Heavily threatened
Oysters have been considered a delicacy since the Stone and Iron Ages. Due to intensive harvesting and centuries of fishing pressure, populations could not recover quickly enough and the population of European oysters increasingly declined. Today, the distribution area of the European oysters still extends in the Atlantic from Norway to Portugal and into the Mediterranean. In the malacological collection of the LIB there are some small, large or even overgrown specimens of both oyster species to admire.

Important for the mudflats
The European oyster, like many other mussel species, plays an important role in the Wadden Sea ecosystem. Due to their high filtration capacity, the shellfish have an enormous purification function for the waters and contribute as mussel beds sediment stabilizing a large part of coastal protection. Oyster reefs are not only a feeding, protection and spawning area, but also a habitat for other species. There are already some projects working to find out if a reintroduction of the European oyster, for example near Helgoland and the Netherlands, would be possible.

Two asymmetrical halves
European oysters consist of a left, lower shell, which is slightly curved, and a right, upper shell, which closes the oyster like a lid. When fully grown, European oysters can grow to about 15 cm in size and live up to 30 years. Shells of oysters found today are often much older. Like all mollusks, they feed on plankton organisms, which they filter from seawater. If an oyster absorbs a grain of sand in the process and is unable to remove it from the shell, it surrounds the grain with a shell of nacre and a pearl is formed.

Reproduction
European oysters are hermaphroditic. They can change sex several times within a life cycle and depending on the water temperature. After fertilization, the small oyster larvae are initially in the mantle cavity of the mother oyster for several days and are then released into the open water. Oyster larvae swim in the sea for a few days and, unlike other bivalves, settle on calcareous hard structures rather than buried in the sand by placing a small blob of cement on the substrate and then dropping in with the lower left shell. They are considered sessile organisms, once they have settled they will stay there for life. In the mudflats they can be found on pilings or on other shells.