The European flat oyster / Native Oyster Ostrea edulis is a habitat-building bivalve mollusc. Young oysters have a planktonic phase, after which they preferentially settle out on adult oysters (Figure 1). Once settled, oysters fuse their shells to the underlying substrate and can therefore form dense aggregations, termed an oyster reef. Oyster reefs provide food and habitat for numerous species and may serve as nursery grounds for some fish species (Figure 2).
Oysters are filter feeders, and a single oyster can filter up to 240 litres of seawater per day. Their filtering activity can improve water quality on local scales (Figure 2). This is not only because oysters remove particles from the water, but also because they then deposit them on the sediment, where conditions for bacteria that break down pollutants such as nitrates are better. This results in enhanced rates of denitrification, a process why which nitrites and nitrates are transformed into inert di-nitrogen gas. By removing particles from the water column the oysters can also increase light penetration to the sediment, and promote the recovery of seagrasses, another threatened and valuable coastal habitat.
Profile of the European flat oyster
|Name:||European flat oyster / European native oyster, Huître plâte, Europäische Auster|
|Scientific name:||Ostrea edulis|
|Size:||up to approx.15 cm|
|Age:||up to 30 years; sexually mature at 3-4 years|
|Appearance:||Shape roundish to oval. Left / lower valve convex, right / upper valve almost flat and fitting inside the left valve to close it. The genus Ostrea ´flat oysters` takes its name from its shape.|
|Habitat:||Estuaries and sea lochs as well as open coastal seas to ~50m depth. Primarily subtidal, colonizing mixed hard substrates, in particular shell material.|
|Range:||The native range is pan-European, including the northeast Atlantic from the south of Norway through to the Mediterranean Sea, as far as the Black Sea (See native range map above, data from Ocean Biogeographic Information System (OBIS)).|
|Status:||Now rare in Europe due to overfishing, impacts of bottom towed gears, and pollution.|
|Legal status across Europe:||At an international scale, the European flat oyster is included in the OSPAR List of Threatened and/or Declining Species and Habitats for the North-East Atlantic (Region II – Greater North Sea and Region III – Celtic Sea). It is also included in Ramsar as “shellfish reefs”, and by some member states as “Reef” in the Habitats Directive.|