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Adriatic Sea (ADRIA); and (iv) the Eastern Mediterranean Sea (EMED) (Zenetos et al.,
2012).
The Western Mediterranean Sea occupies a key position because it receives
the influx of surface waters from the Atlantic, through the Strait of Gibraltar. It is
further compartmentalized into fairly isolated sub-basins with different climatic and
hydrologic conditions. These sub-basins have a different biogeographic character,
which may affect invasion and settlement of aliens. The Alboran Sea, situated
immediately east of Gibraltar, exhibits stronger Atlantic affinities, due to the
continued penetration of Atlantic flora and fauna with the incoming influx of water
(Harmelin and D'hondt, 1993).
The bulk of the Central Mediterranean Sea is represented by the Ionian Sea,
the least known of all the Mediterranean sub-basins (Zenetos et al., 1997). The
Ionian is connected to the Western Mediterranean Sea through the narrow Strait of
Messina, a micro-sector that harbours a wealth of biogeographic peculiarities,
including Pliocene Atlantic remnants and local endemisms (Fredj and Giaccone,
1995), and the larger Strait of Sicily, the meeting point of native Western and Eastern
Mediterranean species (Bianchi, 2007), as well as of aliens of either Atlantic or Indo-
Pacific origin (Coll et al., 2010).
The Adriatic Sea is a rather unique and differentiated area within the
Mediterranean, with a strong contrast between the predominantly linear sandy
shores along the western (Italian) side, and the opposite complex coasts of the
eastern side (Slovenia, Croatia, Montenegro and Albania) forming a maze of islands
and inlets with rocky shores. The hydrographic conditions are also peculiar, with very
low winter temperatures in the northern part, which is also quite shallow (40 m
depth), and very hot summers in the southern part, which is much deeper. All these
features lead to differentiation between the northern and southern Adriatic areas.
The Eastern Mediterranean Sea includes two major bodies of water: the
Levant Sea and the Aegean Sea, together with the smaller Sea of Marmara, which
connects it to the Black Sea. The Levant Sea is warmer than the rest of the
Mediterranean and harbours a significant number of circumtropical species. Atlantic-
Mediterranean elements and Mediterranean endemics are comparatively scarce
(Morri et al., 2009).
Since the construction of the Suez Canal, the Levant Sea is experiencing an
important influx of Red Sea species. Por (1990) defined the geographical limits to the
expansion of Red Sea immigrants in the Mediterranean as the ‘Anti-Psara line’ to the
north (Anti-Psara being an island in the Aegean) and the Strait of Sicily to the west:
these boundaries match the 15 °C surface isotherm for February (Bianchi, 2007).
The Sea of Marmara exhibits peculiar hydrological conditions, with low
salinity waters coming from the Black Sea stratifying over saline waters of
Mediterranean origin on the bottom (Unluata et al., 1990). This hydrological regime
should facilitate the diffusion of Black Sea species into the Northern Aegean rather
than vice-versa, but our knowledge on the exchanges between the two areas is
limited and their biotic affinity is low (Koukouras et al., 2001). In recent times,
climatic change favoured an increase of biotic penetration from the Sea of Marmara
into the Black Sea, which therefore has been undergoing a process of
‘Mediterranization’ (Tokarev and Shulman, 2007).