Native oyster  (Ostrea edulis)

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.