Crassostrea gigas
Information
This species occurs on rocky and muddy bottoms from the intertidal zone down to a depth of about 15 m in shallow, sheltered bays. It can be found in estuaries and coastal sites as it tolerates a wide range of water temperatures (4–35 °C) and salinities (10–40 psu).
Pacific oysters can be hermaphrodites or change sex during their lives. Spawning depends on water temperature and generally occurs at around 18 °C in summer. Fertilization is external and the larval cycle ranges from 3 to 4 weeks. Oyster spat that has settled on hard substrate becomes reproductive after a year.
The Mediterranean native oyster Ostrea edulis differs from the alien C. gigas mainly in that it is usually rounder and flatter and has small teeth on the inner surface of the valves, near the hinge.
Crassostrea gigas is native to the north-western Pacific. Introduced initially to north-western Europe by aquaculture during the industry’s boom in the 1960s, it is still spreading naturally and colonizing sheltered bays and coastal inlets. In the Mediterranean, the species was introduced into the lagoons of the northern Adriatic and into Greece for aquaculture purposes in the 1980s, and it has now established wild populations in the Ligurian, Tyrrhenian, Ionian and Adriatic Seas. Occasional wild colonies of Pacific oysters have also been found in the eastern Mediterranean basin (Greece and Turkey).
Colonization by the Pacific oyster has resulted in ecological competition with native species in many places. At some Mediterranean sites, C. gigas has also formed dense reefs and dramatically changed the original habitat. As a result, it has significantly altered local biodiversity and biomass. The reefs it forms, however, may also provide a rich habitat and refuge for other species and may play an important role in the local marine food web. Imported C. gigas spat and adults from other areas have also introduced several hitchhiking marine species, including seaweeds, pathogens and parasites. Overall, the long-term effects of its explosive invasions are unknown.
The introduction of C. gigas has had a highly significant economic impact as a farmed product. As it has great fecundity, is highly resistant to pathogens and diseases and grows to marketable size more quickly than O. edulis, it is now the most important species in oyster culture. As a side effect, however, the establishment of wild populations of C. gigas might contribute to the decline of native commercial bivalve molluscs, essentially by outcompeting them for food and space.
http://www.ciesm.org/atlas/Crassostreagigas.html
http://www.europe-aliens.org/pdf/Crassostrea_gigas.pdf
Miossec, L., Le Deuff, R-M., and Goulletquer, P. 2009. Alien species alert: Crassostrea gigas (Pacific oyster). ICES Cooperative Research Report No. 299. 42 pp.
