Further evidence of the presence of Caulerpa cylindracea (Bryopsidales, Chlorophyta) and Asparagopsis sp. (Bonnemaisoniales, Rhodophyta) in the Taranto seas (Northern Ionian Sea, Mediterranean Sea), two non-indigenous species (NIS) of macroalgae

The introduction of non-indigenous species (NIS), organisms arriving from beyond their natural (past or present) geographical region and outside of their natural dispersal potential, represents one of the most important threats for biodiversity. This arrival in many cases leads to detrimental community-level modifications and, consequently, to alterations of ecosystem functioning together with socio-economic dimension that is unavoidably affected (Tsirintanis et al., 2022). 1.288 trillion $ are reported as the minimum value of the global economic loss related to biological invasion assessed in the past few decades (1970-2017). The Mediterranean Sea is considered a hotspot for biological invasions counting more than 1000 registered non-indigenous species; of these, seaweeds are approximately 200 to date. Taranto seas, Mar Grande and Mar Piccolo, host 43 NIS of which 15 are macroalgal species; their presence is related to the pathways of introduction which typify the site (Cecere et al., 2016). Here we further report the accidental observation of two NIS considered among the worst in terms of negative impact on biodiversity and ecosystem functioning: Caulerpa cylindracea Sonder and Asparagopsis sp. Caulerpa cylindracea (as Caulerpa racemosa (Forsskål) J. Agardh var. cylindracea (Sonder) Verlaque, Huisman et Boudouresque) was firstly recorded along the coasts of North Africa in the 80’s; the presence of this seaweed in the Gulf of Taranto dates back to 1996. The thalli consist in photosynthetic sub-cylindrical/claviform ramules attached to stolons. After a rapid colonization of the Taranto marine area, due to its impressive invasive ability (stolon elongation approx. 20mm d-1), a regression of coverage was observed almost to its disappearance. In May 2022, only one erect axe was again observed in the Mar Piccolo of Taranto, at 1 meter of depth. In July 2022, a scattered but dense coverage of C. cylindracea was observed colonizing rocks in the intertidal zone, few centimetres of depth in the Gulf of Taranto; at the same site, in September it seemed to be disappeared. In June and July 2022, thalli of Asparagopsis sp. were observed on rocky bottom in the Mar Grande of Taranto, at 1 meter of depth. The genus Asparagopsis comprehends two species A. armata and A. taxiformis featuring a triphasic heteromorphic cycle. The gametophytic stage is represented by pink thalli attached to the substratum with pyramidal structure, and feathery branches; A. armata differs for the presence of harpoon-like branchlets. The tetrasporophyte stage appears as filamentous turfs and was previously considered as a separated species, i.e. Falkenbergia hillebrandii (Bornet) Falkenberg. Moreover, while A. armata is considered genetically uniform, for A. taxiformis molecular analysis showed the existence of 4 different evolutionary lines, two of them present in the Mediterranean Sea (Zanotta et al., 2014). In 1992, the presence of A. armata on the bottom of Cheradi Islands in front of the city of Taranto was reported. The allochthonous lineage of A. taxiformis was probably observed in 2015 in the Mar Piccolo of Taranto (Bottalico et al., 2015). The next step includes the use of molecular analysis on macroalgal samples to correctly identify the species.