Pinctada imbricata radiata
Information
This oyster is found at depths of 5–25 m attached to hard surfaces (natural or artificial) such as rocks, nets, buoys and docks, as well as in seagrass meadows on sandy-muddy sediments, usually in marine habitats with relatively rough hydrodynamic conditions. It may also attach to the shells of the noble pen shell Pinna nobilis or other animals. The oysters attach in clusters of several to many individuals by means of byssal threads, and may form assemblages consisting of pearl oyster shells, worm tubes, algal clumps and anemones, which camouflage the pearl oyster shells. They tolerate a wide range of water temperatures (13–35 °C).
This oyster commonly begins life as a male and when larger than 3 cm it changes sex, becoming female. Spawning occurs in the water column mainly in summer and early autumn, but reproduction can occur all year around. Free-swimming larvae then metamorphose into juvenile pearl oysters.
Pteria hirundo is a native oyster up to 7 cm long. It is easily distinguished by its highly irregular shell shape, in which the valve hinge is straight with two longer asymmetrical lateral ‘ears’. The non-native pearl oyster Pinctada margaritifera can be distinguished by its larger size (up to 20 cm), regular radial ribs arranged in concentric rows and greyish green shell with white or yellowish radial rows of scales.
Originally from the eastern Indian Ocean, Arabian Gulf and Red Sea, the rayed pearl oyster was first recorded in Egyptian Mediterranean waters in the late 19th century, five years after the opening of the Suez Canal (1869). Therefore the primary pathway of introduction was the opening of the canal. Since then the species has successfully spread through the south-eastern basin, while in the western basin it occurs only sporadically. Commercialization of molluscs for mariculture and shipping activities are other reasons for its spread. As larvae can survive in the water column for up to 30 days, they can also be transported in ballast waters, while juveniles and adults can be part of the assemblages encrusting ships’ hulls (fouling).
The gregarious behaviour of P. imbricata radiata in forming extensive oyster beds can modify the structure of habitats and native communities. It may also potentially outcompete other filter-feeding native organisms for food and space. Its broad tolerance of temperatures and air exposure as well as some pollution, and its expanding distribution earn it a place among the worst alien species in Europe (European Environment Agency, 2007).
These have not been quantified but fouling by rayed pearl oysters can affect mussel farms. In the Arabian Gulf, where it is a native species, its economic impact is positive as it is exploited there for its natural pearls.
http://www.ciesm.org/atlas/Pinctadaradiata.html
http://www.europe-aliens.org/pdf/Pinctada_radiata.pdf
Katsanevakis S, et al., 2008. Molluscan species of minor commercial interest in Hellenic seas: Distribution, exploitation and conservation status. Mediterranean Marine Science, 9 (1):77-118.
