The collected thalli at all the above-mentioned sites had green, erect fronds, branched once or twice, 3–4 cm high and 2–4 mm broad; rachis, broad 0.5–1.0 mm, cylindrical to the base and compressed towards the apex, showing oppositely arranged closely adjacent distichous pinnules, seldom tristichous; pinnules,1–2 mm long and 0.2–0.4 mm broad, slightly compressed and curved at the tip; slender creeping stolons were 0.5–1.0 mm in diameter; frequent and short rhizoidal pillars about 3 mm long were also observed.
Specimens from this area corresponded well to the Caulerpa strain previously reported as C. taxifolia from Turkey (Cevik et al., 2007) and C. taxifolia var. distichophylla collected from Southern Sicily (Italy) (Jongma et al., 2013).
The new feather-like Caulerpa population (Fig. 1) covered a surface area of almost 0.6 km2. In the westernmost stations (S. Saba), boulders covered by a deposit of fine sands were colonised, whereas in the easternmost Tono, the amount of fine deposit was negligible. Both living (Acqualadrone) and dead (Tono B), Posidonia oceanica matte were also colonized, the latter in association with Halophila stipulacea. At both colonized sites, a moderate deposit of fine sand was observed. The western border of the Caulerpa formation is sharp, due to an abrupt transition towards a wave-exposed sandy barren area, whereas an irregular transition is observed eastwards. The percentage cover and biomass of C. taxifolia var. distichophylla were homogeneously distributed within each site. The percentage cover (Fig. 3) decreased from 26 % at San Saba to less than 8 % at Tono B (Fig. 3), along to a W–E gradient that was more marked between Aqualadrone and Tono.
The biomass (Fig. 4) followed the same trend as the density, decreasing from S. Saba (69.04 gm−2 ± 2.55 SE) to Tono B (17.91 gm−2 ± 1.37 SE), but showed the highest discontinuity between S. Saba and Acqualadrone (Fig. 4). This gradient, which does not seem related to depth, type or exposure of substratum, follows the prevalent eastwards coastal current, suggesting the associated fine sand deposits might favour the settlement of the slender Caulerpa. Conversely, C. taxifolia colonises the central area of the Messina Strait (Orestano et al. 2001), where fine sedimentation is lacking. Despite the similar morphology, the two taxa are thus ecologically separated and well distinguished.
Ballast water unloading, ship traffic, aquarium trade and anchoring have been suggested as the main vectors of diffusion of C. taxifolia var. distichophylla in the Mediterranea Sea (Jongma et al., 2013; Musco et al., 2014). The current record, which is characterised by a marked discontinuity with respect to the recent report from northwestern Sicily (Musco et al., 2014), as well as from the earlier records from the Strait of Sicily (Jongma et al., 2013), is consistent with a human-mediated dispersal. In this respect, shipping traffic through the Strait of Messina might represent the main vector for the northwards spread of the species. In the newly invaded area, which is a popular destination for tourists but lacks commercial shipping and fishing activities, only the anchoring of touristic vessels can be considered a probable vector of introduction.
In the Tono B site, the alga grows in association with the Lessepsian phanerogam Halophila stipulacea and the native tropical–subtropical green alga Penicillus capitatus Lamarck (Fig. 5). This pattern, which is new for the Mediterranean, reproduces a warm-water association described by Sangil et al. (2010) in the Canary Islands, suggesting a global change-mediated reorganisation of Mediterranean benthic assemblages. This association has not been described in the literature for C. taxifolia var. distichophylla, and involves the co-occurrence of alien and native species, whose dynamics and relationships will be the subject of further notes.
