Native oyster (Ostrea edulis)

Also known as: Common oyster, edible oyster and European flat oyster.

Native oyster (Ostrea edulis)

Native oyster (Ostrea edulis)

Oysters have an interesting yet slightly bizarre life cycle. They almost always start off as male, but then switch backwards and forwards between being male and female. This transformation between sexes can happen twice during one season.

Identification: The native oyster (O.edulis) is a bivalve, meaning that a body is enclosed in a shell hinged by two parts. The shell is oval or pear shaped and has a distinct hooked beak where the two shells join. The two shells are shaped differently, with one being cup shaped and cemented to the substrate, and the other flat, forming a lid (hence why it is sometimes known as the flat oyster). This is a good way of telling it apart from the invasive Pacific oyster (Crassostrea gigas), which was introduced to the UK in 1926 and has a much rougher and elongated shell.

Pacific oyster (Crassostrea gigas)

Pacific oyster (Crassostrea gigas)

Native oysters are usually up to 11 centimetres long and live on average for 5-10 years, but they have been known to grow as long as 20 centimetres and live for 20 years (1).

Where: The native oyster can be found along the western European coast. The main stocks around the British Isles are now on the west coast of Scotland, in the south-east and Thames Estuary, the Solent, the River Fal and Lough Foyle. The native oyster is associated with highly productive estuarine and shallow coastal water habitats. Found in waters up to 80 metres deep, the native oyster attaches to hard substrates, such as rocks, gravel and mud. Once the oyster has attached it becomes immobile and fixed to that position permanently.

Why should we protect it? The native oyster is listed as threatened and/or declining by the Convention for the Protection of the Marine Environment of the North-East Atlantic (the OSPAR Convention). Historically, the native oyster has been a part of European fishing culture for centuries, ever since the Romans began to stock oysters in salt water ponds that they had built (2). However, following heavy exploitation in the 18th and 19th Centuries, the number of oysters surviving to enter the fishing industry market significantly declined and natural oyster beds were destroyed. Although the bulk of oyster production now comes from the introduced Pacific oyster, the value of our native oyster is higher, making it an important species for commercial harvesting. Unfortunately, native oysters are becoming rare. The Pacific oyster, which was introduced to the UK through aquaculture to supplement low native oyster stocks, is out-competing our native species for space and food, and ultimately displacing it (3).

Marine Conservation Zone protection: The native oyster is found in only five of the 31 recommended MCZs under consideration for designation in 2013; Chesil Beach and Stennis Ledges, Kingmere, Poole Rocks, Tamar Estuary. In each of these, the native oyster has been proposed as a ‘Feature of Conservation Importance’ for protection.

The native oyster was also identified as a Feature of Conservation Importance in the Blackwater Estuary recommended MCZ. However, it will not be designated within this site due to data-uncertainty.

References

  1. Natural England. Native Oyster. Available at http://www.naturalengland.org.uk/ourwork/marine/mpa/mcz/features/species/nativeoyster.aspx [Accessed on 16/05/2013]
  2. FAO. Cultured aquatic species information programme. Available at http://www.fao.org/fishery/culturedspecies/Ostrea_edulis/en [Accessed on 16/05/2013]
  3. Natural England. Native oyster beds. Available at http://www.naturalengland.org.uk/ourwork/marine/mpa/mcz/features/habitats/nativeoysterbeds.aspx [Accessed on 16/05/2013]

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