On the occurrence of Uronema marinum (Chaetophorales, Chlorophyta) in the north- western lagoons of the northern Adriatic Sea, Mediterranean Sea (Italy)

We report on the morphology and reproductive features of the green alien macroalgal species Uronema marinum Womersley, a small species of Australian origin, in the north-western Adriatic lagoons in the Mediterranean Sea. It was in summer 2012 that U. marinum was identified for the first time in the lagoon of Venice, in the lagoons and ponds of the Po Delta and in Pialassa della Baiona in Emilia-Romagna. Although Uronema filaments have already been observed from some years, until now they were not noticed because of their small size and similarity with young stages of other filamentous green algae such as Ulothrix implexa (Kützing) Kützing. They differ in determinate growth, curved morphology, basal holdfast, the elongated shape of the cells, a larger diameter towards the apex and their reproductive stages. In the European Atlantic Coasts, from France to Norway, another species is reported: Uronema curvatum, now Okellya curvata (Printz) Leliaert & Rueness (Leliaert et al., 2009), that has a similar shape, but lacks pyrenoids. That species, observed throughout the year, is known from shallow subtidal coasts of the south-western Australia, Micronesia and Hawaii and was probably imported in the lagoons of the Adriatic Sea with the seed of the Manila clam Tapes philippinarum (Adams & Reeve) introduced for economic purposes. Its presence was also correlated to some water column and surface sediment parameters and the presence of the most frequent macroalgae which colonize these environments including some NIS (48 sites sampled both in late spring and autumn). Uronema marinum showed a very high correlation (p <0.001) with Gracilaria vermiculophylla (Ohmi) Papenfuss and significant correlations (p <0.05) with Agardhiella subulata (C.Agardh) Kraft et Wynne, Gracilariopsis longissima (S.G. Gmelin) M. Steentoft, L.M. Irvine & W.F. Farnham and the concentration of reactive phosphorus. Both G. vermiculophylla and A. subulata come from the Pacific Ocean (Zenetos et al., 2010). Uronema marinum, has the same origins and may have been introduced attached on one or both those species. Gracilaria vermiculophylla and A. subulata were also significantly correlated with each other (p <0.001). Uronema marinum is currently widespread in the whole lagoon surfaces, but it is particularly abundant in stagnant and confined areas rich in nutrients where Gracilariaceae and Solieriaceae prevail on Ulvaceae due to the high water turbidity.