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important taxonomic and geographic data gaps concerning Mediterranean alien
biota, and to propose guidelines for the prevention of biodiversity loss caused by
vessel and mariculture-introduced alien species. These are meant to assist the
Contracting Parties to the Barcelona Convention in implementing the SPA protocol
(Barcelona, 1995). The desired goal would be the creation of a new Protocol
specifically aimed at preventing further loss of biological diversity due to the
deleterious effects of the intentional and unintentional introductions of alien
invasive biota in the Mediterranean.

Current status of the Mediterranean alien biota
Although the Mediterranean Sea is only 0.82% in surface area and 0.32% in volume
of the world ocean (Defant, 1961), the marine organisms which dwell in this sea
represent 4-18% of the world marine species, depending on the phylum taken into
consideration (Lejeusne et al., 2010). The dominant group among alien species in the
Mediterranean sea is Mollusca (with 215 species), followed by Crustacea (159) and
Polychaeta (132) (Zenetos et al., 2012) (Fig. 6).
One reason for the high number of species present in the Mediterranean Sea
might be its tormented geological history, which has led to high rates of
environmental change and speciation. Another reason is the variety of climatic and
hydrologic situations to be found in this sea, leading to the co-occurrence of cold,
temperate and subtropical biota (Bianchi and Morri, 2000). Finally, the rate of
endemism i.e. the number of species living exclusively in the Mediterranean is
relatively high, averaging more than one-quarter of the whole Mediterranean biota
(Tortonese, 1985; 1992; Giaccone, 1999; Lejeusne et al., 2010).

Figure 6. Contribution of marine alien taxa in the Mediterranean Sea (Zenetos et al., 2012).

The high marine biodiversity could be the result of its geological history (Maldonado,
1985; Stanley and Wezel, 1985; Ruffel, 1997), which has led to a rate of