The invasive brown seaweed Rugulopteryx okamurae (Dictyotales, Ochrophyta) continues to expand: first record in Italy

The brown seaweed Rugulopteryx okamurae (Dictyotales, Ochrophyta), native to the Pacific Ocean and widely distributed in Asia, has been recently recognized as an emblematic case of biological invasion by marine macroalgae in European waters. Since 2015 and from the Strait of Gibraltar, R. okamurae has rapidly spread towards Atlantic and Mediterranean coastal areas exhibiting an invasive behaviour with significant ecological and socio-economic impacts. Here, we report the first morphologically and genetically confirmed observation of this species in Italy, along the north-western coast of Sicily (Gulf of Palermo). Specimens were found as drifted material and as an established population on Posidonia oceanica, representing its new eastern distribution limit in the Mediterranean Sea, as the species was previously established in Marseilles (France). Furthermore, we performed a favorability distribution model with the current introduced distribution of the species for the Mediterranean, which shows most of the western Mediterranean, including the Balearic archipelago, Corsica and Sardinia, central Mediterranean, including Sicily, and the northern coast of Africa together with eastern Mediterranean basin, as highly favorable for R. okamurae. The most probable vectors for this introduction are sea currents and maritime traffic, including fishing activities. This hypothesis was supported by some of the ranked variables in the favorability model, i.e., current velocity, and the proximity of the introduced population to fishing ports. These results are a warning that the species can cover large sea distances via sea currents, thus also threatening the ecosystems and marine resources of the central and eastern Mediterranean Sea, as these areas are forecasted to be highly favorable regions for the species. We suggest coordinated actions at the European level with stakeholders in the fishing sector regarding prevention, both because it is a highly affected sector and because it potentially has a very important role in the dispersion of the species.