The status and trends of exotic Molluscan in the Mediterranean Sea

• Argyro Zenetos.
• HCMR – Hellenic Center of Marine Research – Saronida – Greece.
• Email: zenetos@ncmr.gr
  
• The Regional Workshop on Marine Sciences & Natural Resources 25-26 May 2004.

About 200 bivalvia species which were recorded as exotics in the Mediterranean between 1870 and 2002, were examined in the CIESM atlas. Of these only 137 fulfilled the CIESM criteria and considered as valid exotics into the Mediterranean. The rest, among which 2 cryptogenic species [Littorina saxatilis (Olivi, 1792) and Teredo navalis (Linné 1758)], were excluded.

The process of introductions has become unprecedented in magnitude since the 1970s and is not slowing down. Ten more species are known to have been recorded since middle 2002 especially along the Israel, Lebanon and S. Turkey coasts.

The main pathway for the arrival of exotic marine molluscs into the Mediterranean is Suez Canal followed by shipping and aquaculture. Among the 137 established or alien exotic molluscs of the Mediterranean treated in the Atlas, only 17 appear to have been transported via shipping, while aquaculture (intentional or accidental transport of live molluscs accounts for 12 species. The bulk of the introduced molluscan species (118 species), are species of Indo-Pacific origin found mainly in the eastern basin of the Mediterranean. Among these species, some are most likely to have spread by their own means "lessepsian immigrants" while for others the intervention of transport by ship hulls or ballast water can be suspected.

The mode of introduction remains unknown for a good number of species. This is further complicated by the fact that some intentionally imported species have established wild populations and have become locally invasive. One such case is the pearl oyster Pinctada radiata, which is one of the 10 invasive molluscan species in the Mediterranean and presents a wide distribution in the eastern basin. The mode of its introduction, along Greek, Syrian and Tunisian coasts, is currently under investigation in the framework of UNEP/MAP funded project.. Preliminary results have shown morphological differences among the populations of the three study areas. However, genetic analyses has revealed absence of differentiation among the populations of the three study sites. Absence of polymorphism in the totality of samples in all enzymes was observed with the only exception being a single individual from Salamina population (Greece) which presented a different haplotype for TaqI enzyme. Analyses of larger sample size and comparisons with other populations is expected to reveal the true mode of introduction.