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Mediterranean invasive species factsheet

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.

Economic impacts
The introduction of C. gigas has had a highly
Ostrea edulis. Photo: H. Zell
significant economic impact as a farmed product. As
it has great fecundity, is highly resistant to pathogens
Similar species and diseases and grows to marketable size more
The Mediterranean native oyster Ostrea edulis quickly than O. edulis, it is now the most important
differs from the alien C. gigas mainly in that it is species in oyster culture. As a side effect, however,
usually rounder and flatter and has small teeth on the establishment of wild populations of C. gigas
the inner surface of the valves, near the hinge. might contribute to the decline of native commercial
bivalve molluscs, essentially by outcompeting them
Small teeth present for food and space.

Management options

Suggested prevention actions to avoid Pacific
oysters becoming established in the wild should
include education and public awareness-raising,
together with a monitoring program to control
invasive species associated with ballast water,
marinas and aquaculture. Monitoring also helps in
Ostrea edulis
detecting colonies early and eradicating or containing
them before further spread occurs. Aquaculture
Brief history of its introduction and farms near MPAs should be encouraged to cultivate
pathways native species or use sterile triploid seed oysters.
Crassostrea gigas is native to the north-western Control action is feasible only in particular
Pacific. Introduced initially to north-western Europe by conditions, such as where many individuals are
aquaculture during the industry’s boom in the 1960s, localized in a very restricted area and, if possible,
it is still spreading naturally and colonizing sheltered before spawning occurs. Large-scale oyster removal
bays and coastal inlets. In the Mediterranean, the experiments with mussel dredges have been
species was introduced into the lagoons of the conducted in the Netherlands with limited success.
northern Adriatic and into Greece for aquaculture Prior to any control actions, an environmental impact
purposes in the 1980s, and it has now established assessment of the control procedure should be
wild populations in the Ligurian, Tyrrhenian, Ionian carried out, as in the case of Chama pacifica or
and Adriatic Seas. Occasional wild colonies of Pacific Spondylus spinosus.
oysters have also been found in the eastern
Mediterranean basin (Greece and Turkey).
Further reading
http://www.ciesm.org/atlas/Crassostreagigas.html
Ecological impacts http://www.europe-aliens.org/pdf/Crassostrea_gigas.pdf
Colonization by the Pacific oyster has resulted in Miossec, L., Le Deuff, R M., and Goulletquer, P. 2009. Drawings: Juan Varela
ecological competition with native species in many Alien species alert: Crassostrea gigas (Pacific oyster).
places. At some Mediterranean sites, C. gigas has ICES Cooperative Research Report No. 299. 42 pp.
also formed dense reefs and dramatically changed

68 Monitoring Marine Invasive Species in Mediterranean MPAs: A strategy and practical guide for managers