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Figure 8. Number of marine alien species per major groups in the Marine Strategy Framework
Directive subregions of the Mediterranean Sea (Zenetos et al., 2012).

The principal pathway of crustacean introduction varies according to the subregion.
In the Eastern Mediterranean Sea almost 80% are derived from the Indo-Paciic
through the Suez Canal, although in some cases these inputs can be dual (corridors
and shipping, either in ballast water or among hull fouling) or even caused by
aquaculture. In the Western Mediterranean Sea the situation is different, as a
considerable proportion of non indigenous species (between 57% and 71%) has been
introduced by shipping, 24% to 33% used corridors as a primary pathway (Suez
Canal, and in a few cases inland canals), and only 10% to 14% can be linked to
aquaculture. The increase of maritime traffic is an important pathway for
introduction and dispersal of alien decapod species, since larvae can survive long
periods in ballast water (Mizzan, 1999; Occhipinti Ambrogi, 2000). The presence of
non indigenous species populations in some Mediterranean areas can also be related
to their trade: Necora puber and Paralithodes camtschaticus (Faccia et al., 2009) are
quite frequently found alive in the markets.
Larval crab stages (zooea, megalopae) have been found in ship’s ballast but
are by no means common, and crabs are rarely on hulls. Adult crabs have been
found in bottom sediments in ballast tanks and in sea chests and other areas not
routinely affected by ballast water management (Grosholz, 2011). Once introduced
to a new continental margin, crabs may frequently expand their range through
dispersal of planktonic larval stages by advection of ocean currents. Several
introduced crab species have been rapidly dispersed by ocean currents along a
coastline following an initial human-mediated introduction to a new continental
region.
Many crab species have the tendency to expand their native range
significantly into areas that are considerably outside of their typical range. This is
partially caused by their long planktonic development periods, during which
developing larvae may be carried many hundreds of miles by ocean currents. Among
the species that best exemplify this pattern are the swimming crabs of the genus
Callinectes, which include the commercial blue crab Callinectes sapidus.
Once introduced to a new region, many crabs can rapidly expand their range
along the coastline. These dispersal events include some of the fastest range
expansions recorded for any introduced species. Among the most rapid range