Page 12 - Mannino_Balistreri_2017

232 Mannino & Balistreri: An updated overview of invasive Caulerpa taxa ...

Klein & Verlaque 2008; Holmer & al. 2009; Mannino & Di Giovanni 2011; Matijević &
al. 2013). Therefore, it can be regarded as a “habitat modifier” (sensu Wallentinus &
Nyberg 2007), ‘‘ecosystem engineer’’ (sensu Jones & al. 1994) and ‘‘foundation species’’
(sensu Bruno & al. 2003). Recently, it has been hypothesized that the bacterial community
associated with C. cylindracea could have a potential role in its spread (Rizzo & al. 2016).
Fragmented or less structured habitats such as dead mattes of P. oceanica, sparse P.
oceanica and C. nodosa meadows, algal turfs and annual photophilic macroalgal assem-
blages seem to be more vulnerable to invasion by C. cylindracea (Ceccherelli & al. 2000;
Montefalcone & al. 2007; Katsanevakis & al. 2010). Recently, Bulleri & al. (2016)
observed that the effects of C. cylindracea are more negative in well preserved sites.
Usually, C. cylindracea showed a marked seasonal cycle, with an alternation of a long
growth period, from spring to autumn, and a withdrawal (resting period) in winter.
Gametogenesis has never been observed in Sicily and in circum-Sicilian Islands
(Giaccone & Di Martino 1995; Di Martino & al. 2006; Mannino & Di Giovanni 2011).
Since gametogenesis has been locally observed (e.g. in the South Aegean Sea;
Panayotidis & Žuljević 2001), it is likely that sexual reproduction may have contributed to
its rapid spread in other parts of the Mediterranean Sea.
Caulerpa taxifolia (invasive aquarium strain)
Since it was officially reported exclusively for the coasts of the Strait of Messina and of
Favignana Island so far (Fradà Orestano & al. 1994; Orestano & al. 2001; Profeta & al.
2004; Gianguzza & al. 2006a, b), its spread was very limited if compared to that of C.
cylindracea. Its presence along the coast of Favignana Island is likely to be a consequence
of a secondary anthropogenic introduction (Gianguzza & al. 2006a, 2006b).
According to Di Martino & Giaccone (1996) in the Strait of Messina the complex C.
taxifolia - Caulerpa mexicana Sonder ex Kützing would be present. Chisholm & al. (1995)
stated that the Mediterranean C. taxifolia and the Eastern Mediterranean C. mexicana
would be conspecific, suggesting its occurrence in the Mediterranean as a result of a bio-
geographic dispersion instead of an accidental introduction. But later, Jousson & al. (1998)
demonstrated that the aquarium-Mediterranean strain of C. taxifolia was not related to any
specimens of C. mexicana, thus invalidating the hypothesis of Chisholm & al. (1995).
A combination of unfavourable intrinsic (e.g. lack of specialized propagules, low prop-
agation speed and/or weakness of the population) and/or extrinsic factors (e.g. hydrody-
namics conditions and lack of shipping-trade with the other parts of Sicily) could have
stopped its northward and southward expansion and could be also responsible for the
regression observed in the Strait of Messina between 2008 and 2009 (Meinesz & al. 2010).
The regression and/or disappearance of C. taxifolia populations observed in other
Mediterranean areas (Iveša & al. 2006; Montefalcone & al. 2015) has also been related to
genetic degeneration of the Mediterranean clone and to the presence of parasites
(Longepierre & al. 2005; Meinesz & al. 2010). As in other Mediterranean areas, sexual
reproduction remains unknown. Transport-stowaway is considered the plausible pathway
of introduction for local specimens.
Caulerpa taxifolia, which was also considered established in Sicily, did not behave as a
highly successful and fast-spreading species as it did in other Mediterranean areas
(Boudouresque & al. 1992; Meinesz & al. 2001; Balata & al. 2004) and as we would have
expected from the “alga killer”.