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Caulerpa taxifolia var. distichophylla
As in other Mediterranean regions, it mainly occurs in shallow waters where it can be more
easily ripped off and spread (Schembri & al. 2015). Even if it has been recently introduced, it
is already established in the area and it is considered a potential threat for the indigenous com-
munities but also for fishing activities, to such an extent that it is considered a real pest (Musco
& al. 2014). It has been observed that winter water temperatures might limit its spread to the
northern and western parts of the Mediterranean Sea (Jongma & al. 2013).
The occurrence of this alga in association with phanerogams and green algae of tropi-
cal-subtropical origin (e.g. H. stipulacea and Penicillus capitatus Lamarck) would suggest
a reorganisation of Mediterranean benthic communities as a consequence of global change
(Picciotto & al. 2016). Fertile thalli have not been observed in Sicily so far. Also for C. tax-
ifolia var. distichophylla the Sicilian coast would be the start of an invasion in the
Mediterranean Sea (Musco & al. 2014). Transport-stowaway is considered the plausible
pathway of introduction for local specimens.
Sicily, representing the western and northern limit of this taxon (Jongma & al. 2013;
Musco & al. 2014), would have been the source of the Maltese populations of C. taxifolia
var. distichophylla. Since the traffic of recreational and/or commercial vessels is very
intense between the two islands, shipping is the most likely vector (Schembri & al. 2015).
As hydrodynamics conditions do not facilitate the westward and northward spread, sec-
ondary shipping could be the vector of its spread along the northern coast of Sicily.

Conclusions

In the Mediterranean Sea, maritime traffic plays an important role in the introduction
and spread of alien species, and the Pacific Ocean is the main donor of alien species due
to climatic affinities. Since the Suez Canal is considered to be one of the main pathways
for the introduction of marine alien species, the creation of a second canal, increasing the
number of introductions, will certainly have a strong impact on the Mediterranean ecosys-
tems (Galil & al. 2015). Sicily and circum-Sicilian Islands, at the crossroads between the
western and eastern Mediterranean and interested by intense maritime traffic, are particu-
larly vulnerable to biological marine invasions. The fact that many colonized areas were
located in the vicinity of harbours, and exposed to human activities (e.g. shipping, tourism,
fishing), would support this hypothesis.
Indeed, maritime traffic can produce a constant spillover of new invaders into surround-
ing areas, making this area an important transit zone and a source for secondary dispersal,
highlighting the key role it can play in the circulation of alien species within the
Mediterranean Sea. In particular, as highlighted in this study, the southern and south-east-
ern coasts, bordering the Strait of Sicily, would deserve particular attention.
The three invasive taxa showed different spread dynamics. Caulerpa cylindracea and
C. taxifolia var. distichophylla behaved as fast-spreading algae whereas C. taxifolia did not
behave in the same manner. Divergent spread dynamics between C. cylindracea and C.
taxifolia were already highlighted by Montefalcone & al. (2015) in the Ligurian Sea. As
the authors stated, it is not possible to generalize the future behaviour of invasive species
in the Mediterranean Sea. Moreover, the evaluation of the spread dynamics of invasive