This bivalve mollusc is an important commercial species. Common in Strangford Lough during the 19^th century, it became scarce in the Lough in the early 20^th century due to overfishing. Commercial restocking during the late 1990s has led to the oyster being found more widely there.

In brief

• In Northern Ireland the oyster is almost exclusively found in Strangford Lough
• It is generally found in shallow waters at all times of the year
• Listed as a UK Priority Species
• The flat oyster is sensitive to various factors such as substratum loss, smothering, non-native species, heavy metal contamination, changes in oxygenation, etc.

Species description: The shell is solid, oval or pear-shaped, or circular when young, inequivalve with the lower valve convex, the upper valve flat. The lower valve has coarse concentric sculpture, but lacks ribs, and is used to attach to the surface. The shell colour is off-white, yellowish or cream, with fawn or bluish-purple concentric bands on the upper valve. The periostracum is thin, dark brown. The interior of the shell is pearly white or bluish-grey, often with darker blue areas marking enclosures of detritus; the adductor muscle scar is white or yellowish-white. The shell is up to 11cm long, rarely larger, but there can be considerable variation in shape and distortion of shells due to circumstances of habitat. Mother-of-pearl is secreted by the oyster around any foreign body that gets trapped between shells, for example a piece of sand or grit. In time, this builds up and forms a pearl.

Life cycle: This species matures as male at about three years old. After the first spawning, it becomes a female and then continues to switch after each reproductive cycle, controlled by water temperature (protandrous hermaphrodite). Eggs are fertilized within the gills and mantle cavity of the animal, and retained 7-10 days until they have developed to the veliger stage and released. Spawning takes place from June to September, with planktotrophic development. Spat settlement takes place after 10-20 days. Oysters normally live from 5-10 years, but up to 15 years.

Similar species: There is only one similar species, the introduced Portuguese oyster Crassostrea gigas. Ostrea edulis has 10 fine ribs on the left valve only, and a white, off-white or cream adductor muscle scar. Both valves of Crassostrea gigas have 6-7 prominent ribs, and the adductor muscle scar is deep purple or brownish.

How to see this species: This species is most easily seen in northern half of Strangford Lough, normally on the lower shore and shallow sublittoral on comparatively firm bottoms of mud, rocks, boulders, muddy sand, muddy gravel with shells, hard silt, old peat bottoms or on man-made spat collectors. It is normally in less than 20m, in water rich in plankton, as it is an active suspension feeder. It is predated by starfish, sting winkles and the common whelk. It is present at all times of the year.

Current status: In Northern Ireland, the flat oyster is largely localised in Strangford Lough. Until the mid-1990s, it was only seen occasionally from the shore to 23m. Since re-seeding there, it has been seen more frequently, particularly in the northern part of the Lough. Extensive oyster beds are known to have existed in Carlingford Lough for several hundred years; there were beds of oysters in Lough Foyle and Strangford Lough, but most stocks crashed during late nineteenth or early twentieth century, and oyster fishing ceased — suspected to be result of overfishing. A small population existed in Strangford Lough, possibly limited from expansion by availability of suitable substratum, and wide separation of breeding adults. The species was therefore considered to have declined, and to be scarce. Between 1997 and 1999, the EU funded a project to re-establish a sustainable native oyster fishery in Strangford Lough, using seed and adult brood-stock from a disease-free population in Lough Foyle. This has increased the prevalence of the species in this area. The flat oyster is commonly found around the west coast of Ireland, with many old records, particularly from the east coast. It is also widely distributed around Britain from the lower shore to 83m and elsewhere from the Norwegian Sea south to Iberian peninsula, Atlantic coast of Morocco, Mediterranean and the Black Sea as indigenous populations or introduced stocks.

Why is this species a priority in Northern Ireland?

• Listed as a UK Priority Species
• It has undergone decline and is scarce.

Threats: The flat oyster is sensitive to substratum loss; smothering; synthetic compound contamination; introduction of microbial pathogens/parasites, non-native species; extraction of species; desiccation; increase in emergence time; decrease in temperature; increase in wave exposure; abrasion and physical disturbance; heavy metal contamination; changes in oxygenation. The main threats are disease (Bonamiosis from parasitic protozoan Bonamia ostreae), parastites such as the copepod Mytilicola intestinalis, and introduced species American oyster drill Urosalpinx cinerea (predator) and the slipper limpet Crepidula fornicata (competitor for space and food), neither of which is currently present in Northern Ireland. Severe winters such as 1947 and 1963 also increase oyster mortality. It is thought that an increase in sea temperatures would favour consistent and intense settlements of oysters in existing oyster areas, and significant natural settlements of oysters would evolve elsewhere.

Distribution Map from NBN: Ostrea (Ostrea) edulis at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Ostrea (Ostrea) edulis at iNaturalist World Species Observations database.

WoRMS: Ostrea (Ostrea) edulis at World Register of Marine Species. Accepted name: Ostrea edulis Linnaeus, 1758. AphiaID: 140658.

Classification: Biota; Animalia; Mollusca; Bivalvia; Autobranchia; Pteriomorphia; Ostreida; Ostreoidea; Ostreidae; Ostreinae; Ostrea