Native oyster (Ostrea edulis)

KingdomAnimalia
PhylumMollusca
ClassBivalvia
OrderOstreoida
FamilyOstreidae
GenusOstrea
SizeShell width: up to 11 cm

Not protected. Listed as a priority species under the UK Biodiversity Action Plan.

The native oyster is a bivalve mollusc, which means ‘two shells’, and is rough, scaly and yellowish-grey in colour. Each valve differs in shape and size; the left one (the one used by the oyster to attach itself to a surface) is concave, while the right one is flat and fits snugly inside the left. The right valve has concentric rings of a bluish colour, and the whole animal is roughly pear-shaped. Inside the shell, the colours range from blue to grey and include the opalescent ‘mother of pearl’. Mother of pearl is secreted by the oyster around any foreign body that gets trapped between the shells, for example, a piece of sand or grit. In time, this builds up and forms a pearl.

Oysters are found widely around the western European coastline as far north as Spitsbergen, and south to Morocco and the Mediterranean. They can turn up all around the British coast with the best areas being the Thames Estuary, the west coast of Scotland, the Solent, the estuary of the River Fal, and Loch Foyle. They are also cultivated in other parts of the world such as North America, Japan and Australasia.

Oysters need a firm bedrock or an artificial equivalent along coastlines on which to fasten themselves, in water rich in plankton no deeper than 20 metres.

Native oysters are gregarious animals, and start their lives as males. They mature sexually as males between eight and ten months old. From then on, oysters will change sex regularly, depending on the water temperature. If the temperature reaches 16°C, they become females every three or four years. If the temperature reaches 20°C, they will change to females each year. They only revert to being males during the cooler intervening periods. Oysters may live for as long as 15 years but the usual lifespan is thought to be around six years.

Eggs are stored and fertilised in the gill cavity of the female and remain there for a week before becoming free-swimming larvae and being released. The sperm is passed through the gills as part of the normal feeding process. The oyster larvae join the plankton in the open sea until, after 10 or 20 days, they find a surface to attach themselves. Adult oysters feed by filtration, sieving out the plankton using their gills.

The towns of Colchester in Essex and Whitstable in Kent have become famous for their Oyster Festivals. Oysters have been an important food source since prehistory, and during the Roman occupation, British oysters were exported in large quantities back to Italy. One claim for eating them is that they act as an aphrodisiac, although there is no scientific proof for this argument.

Once a plentiful species around the UK coastline, the native oyster was the victim of serious overharvesting during the late 19th century, as they were a staple food of the poor and working classes. In the 15th century, fourpence would buy eight gallons of oysters, but by the beginning of the 20th century, they had become the expensive luxury they are today. This overfishing was probably a consequence of the expanding townships following the Industrial Revolution.

Today the principal threat to the wild native oyster comes from disease and two introduced species, the American oyster drill shellfish Urosalpinx cinerea, and the slipper limpet Crepidula fornicata. The disease, Bonamiosis, is spread by a parasitic protozoan Bonamia ostreae. The oyster drill was brought to the UK by accident with imported American oysters and feeds on the young shellfish. The slipper limpet forms dense beds, competing with the oyster for spaces and food resources. It also produces a material known as ‘mussel mud’, a substance that covers potential oyster beds and makes it difficult for young oysters to establish themselves on a firm surface.

The oyster industry in the UK is a lucrative one, but is now dominated by the introduced Japanese or Pacific oyster Crassostrea giga. However, native oysters are farmed and potential sites are sometimes prepared by dumping broken shells, an aggregate known as ‘cultch’, which encourages young oysters, called ‘spat’, to form new beds.

The native oyster is listed as a priority species in the UK Biodiversity Action Plan. The shell fishing industry in the UK is carefully regulated, and there is also a closed season on harvesting from 14 May to the 4 August during the critical spawning period, although this does not cover farmed oysters. The principal aims of the UK Action Plan for the native oyster are to maintain the current range of the oyster around the UK coastline and, where possible, increase the population and the number of viable oyster beds. In order to improve the species’ chances, a number of laws and directives have been introduced in recent years. In 1987, a ban was imposed preventing the use of TBT-based anti-fouling paints on all vessels less than 25 metres in length. The ban was introduced for these smaller vessels as they are more likely to come into the shallower coastal waters than the larger sea-going ships. Shellfish farmers have welcomed the banning of the use of this paint, which is believed to affect the reproduction rates of oysters. There is also a European Directive governing the spread of diseases prevalent among bivalves. The shellfish industry is being encouraged to conduct more environmental impact assessments in areas thought suitable for re-introduction or, in some cases, on former sites that have become derelict.

For more on the native oyster see the MarLIN species fact sheet, available on-line at:
http://www.marlin.ac.uk/species/Ostreaedulis.htm

Information supplied by English Nature.
http://www.english-nature.org.uk