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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
expansions is that of the European green crab, which spread along the western coast
of the United States at rates of 200 km per year. Also, the crab Hemigrapsus
sanguineus experienced rapid range expansion on the eastern coast of the United
States. In Europe, Eriocheir sinensis also spread rapidly along the coast and
throughout many river systems during the early twentieth century (Grosholz, 2011).
Rapid spread is by no means the rule or even the norm for the same species. For
instance, Carcinus maenas has spread very little in southern Australia and Tasmania,
and the rates of spread in eastern North America were very minimal for long periods.
The sequential stages of an invasion process according to (Walther et al.,
2009), start from the introduction of a few precursor individuals, which only
temporarily occur in a site during short favourable climatic periods or are spatially
restricted to favourable micro-habitats (Fig. 4). Continued climatic warming might
then prolong the duration of these occasional occurrences of initial introductions,
increase their frequency or enlarge the range and area of suitable habitats, making it
more likely for these species to persist, to occur more frequently and to develop
larger populations. With further global warming, alien species originating from
warmer regions could build up numerically and spatially larger populations that
might spread to wider areas.