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Mediterranean, 249 in the central Mediterranean, 190 in the Adriatic Sea and 308 in
the western Mediterranean) (Fig. 7).

Figure 7. Number of non indigenous species introduced per decade, according to major groups, in the
Mediterranean Sea (Zenetos et al., 2012).

Crustacean alien species in the Mediterranean Sea
Only 37 alien crustaceans were known before 1950s in the Mediterranean. Between
the 1950s and the 2000s, further 122 species were recorded, which represents an
increase of 327%. The highest increase in decapod non indigenous species occurs in
the Eastern Mediterranean Sea (Zenetos et al., 2012).
The highest proportion (78%) of crustacean non indigenous species occurs in
the Eastern Mediterranean Sea, being 28% in the Western Mediterranean Sea, 23%
in the Central Mediterranean Sea, and 14% in the Adriatic Sea (Zenetos et al., 2012)
(Fig. 8). The rate of increase of alien crustacean species in the Mediterranean Sea is
currently 3-4 species per year. Approximately 60% of the known alien brachyuran
species worldwide have been reported from the Mediterranean, which makes it the
richest sea in terms of invasive crabs (Brockerhoff and McLay 2011).
The status of some species with an eastern Atlantic origin in the Western
Mediterranean Sea is difficult to determine, mainly because of intense fishing and
transport activities that occur between Africa and Spain or France (Luque et al.,
2012). Furthermore, the Strait of Gibraltar is a boundary more or less permeable to
Atlantic species that naturally increase their distribution range. Finally, there is a
limited knowledge on the biodiversity from North African littoral. Therefore a
number of species reported in the Western Mediterranean Sea considered to have
expanded their geographic range, could in fact have been introduced by man.
(Zenetos et al., 2012).