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   Ostrea edulis (mollusc)   
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    Taxonomic name: Ostrea edulis Linnaeus, 1758
    Synonyms: Ostrea adriatica Lam-Middendorff 1848, Ostrea taurica Krynicki 1837
    Common names: common oyster (English), edible oyster (English), eetbare oester (Dutch), Essbare euster (German), European flat oyster (English), European oyster (English), huître comestible (French), huître plate Européenne (French), istiride (Turkmenistan), native oyster, oester (Dutch), ostra Europa (Spanish), platte oester (Dutch), stridia (Bulgaria), stridie (Romania), ustritsa (Russia)
    Organism type: mollusc
    Ostrea edulis (the European flat oyster) is native to Europe and the Mediterranean. It has been introduced to the northwestern Atlantic Ocean for aquaculture. Usually found in muddy areas, O. edulis has long been harvested for food. Over-harvesting in its native range, however, has caused it to be reintroduced to Europe. While in Atlantic waters, O. edulis became infected with the disease bonamiasis (Bonamia ostrae). This disease has caused widespread mortality in the vast majority of O. edulis.
    Ostrea edulis is a hermaphroditic (Mirella de Silva et al. 2005) bivalve mollusk with a distinct beak that is patterned with delicated foliation. O. edulis can grow up 20cm or more and live up to 20 years. O. edulis is ovular and has a rough scaly surface with two distinct halves. The two halves of the shell are different shaped; the left shell is concave and attached to the substratum and the right is flat with tough edges and acts as a lid for the left shell. Its right valve is cream-coloured with light brown or blue circular bands. The inner surface of O. edulis is smooth and white or bluish-grey and shiny with some dark blue spots. At the narrow ends of the shell are stretch ligaments that hold the shells together. "A large central muscle serves to close the valve against the pull of the ligament." The outer shell is composed of flaky layers which may include laminar and hollow chambers. O. edulis contains a creamy beige to pale grey meat that is slightly salty or bland in taste. The meat's texture can be tender or firm. (FIGIS(a), 2006)
    Similar Species
    Crassostrea virginica

    Occurs in:
    coastland, estuarine habitats, marine habitats, riparian zones
    Habitat description
    Ostrea edulis prefers the firm bottoms of mud, rocks, muddy sand, muddy gravel with shells, hard silt, and artificial habitat created with broken shells or "culch" (Jackson, 2003). O.edulis can be found in muddy areas attached to hard surfaces at depths of 30 ft (9.144 metres) (CZM, undated).
    General impacts
    Ostrea edulis stocks that were introduced to the northwest Atlantic coast suffered high mortalities from bonamiosis (by the protistan parasite Bonamia ostrea). These were re-stocked from other parts of Europe. The original outbreak of bonamiosis is blamed on a consignment of O. edulis being brought in to France from California, USA (Carolyn Bromley pers.comm March 2013)
    O. edulis has been harvested for 6,000 years (Diaz-Almela et al. 2004). O. edulis when abundant was often eaten by Europeans, however a decline in numbers have made them a delicacy (Weichtiere, undated).
    The bacteria Nocardia crassostreae infects O. edulis and makes it heat sensitive. Oyster herpesvirus infects O. edulis and results in larval and seed mortality (Ruesink et al. 2005). O. edulis is threatened by two introduced species in its native range, Urosalpinx cinerea (drill shellfish) and Crepidula fornicata. The United Kingdom has listed O. edulis as a priority species in the UK Biodiversity Action Plan (ARKive, 2006). Between 1957 and 1959, 16,000 of this species was imported from Conway, North Wales, to St. Andrews and Ellersie, Prince Edward Island (Vercaemer et al. 2003).
    Geographical range
    Native range: Norway, Morocco, Mediterranean Sea, Black Sea, France, Italy, Greece, UK, Germany, Ireland, Sweden, Denmark, Croatia, Ukraine, Portugal, Spain, Netherlands (Diaz-Almela et al. 2005; Ruesink et al. 2005; FIGIS(a), 2006; FIGIS(b), 2006; Johnson et al. 1999; Kennedy & Roberts, 1999; Jackson, 2003)
    Known introduced range: Japan, Tonga, Fiji, US, Canada, Namibia, Israel, Mauritius, New Zealand, South Africa (Carlton, 1992; FIGIS(b), 2006; CZM, undated; Ruesink et al. 2005; Ray, 2005).
    Management information
    The management of O. edulis is vital to controlling the spread of diseases (FIGIS(a), 2006). A disease of concern is the protist Bonamia ostrae which causes a 70-80% mortality rate in O. edulis. Another protist, Marteilia refringens causes 75-100% mortality in O. edulis. Mikrocytos mackini also affects the oyster (Ruesink et al. 2005). When importing O. edulis, it should be transported from countries where no disease has been present for the past 2 years. The Office International des Épizooties (OIÉ) set forth guidelines to ensure that no parasites are detected while under an official surveillance program for two years. European zoosanitary directives implemented a zoning system to restrict the spread of diseases. Site selection and density reduction are a major focus in matters concerning management (FIGIS(a), 2006). In the United Kingdom efforts to manage O. edulis involve maintaining the abundance of stock at the fisheries while decreasing local densities to limit the spread of disease, especially Bonamia ostrae (UKBAP, 1999).
    Ostrea edulis is a filter feeder that eats microalgae or phytoplankton. Autotrophic flagellates and diatoms are also important food for O. edulis (Jonsson et al. 1999).
    Ostrea edulis is a protandric hermaphrodite that changes sexes twice during one season. They are males early in the spawning season and females later and vice versa. During reproduction, female gametes are released into the palleal cavity where they are fertilized by externally released sperm (FIGIS(a), 2006) which is passed through the gills as part of the normal feeding process (ARKive, 2006). Females produce between 500,000-1,000,000 fertilzed eggs per spawning period. The eggs are incubated for a period of 8-10 days (depending on temperature) and released into the environment (FIGIS(a), 2006). In their native range, O. edulis spawns between late June and mid-September. Young oyster spat can be seen from late summer on. (Kennedy & Roberts, 1999).
    Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
    Last Modified: Monday, 14 May 2007

ISSG Landcare Research NBII IUCN University of Auckland