Shellfish import and hull fouling as vectors for new red algal introductions in the Venice Lagoon

Touristic and commercial shipping traffic, the import of fish products and aquaculture activities are the most important vectors for the introduction of non-indigenous species (NIS) in the Mediterranean Sea. In this context, the Venice Lagoon (North Adriatic Sea) is considered a hotspot of NIS introductions. Among macroalgae, identification of these alien taxa on the basis of morphological characters is difficult due to their simple morphology, phenotypic plasticity and the occurrence of morphologically overlapping species. Here we report three new introduced species for the Mediterranean Sea: Aglaothamnion halliae, Melanothamnus japonicas and Polysiphonia schneideri. The samples belong to different genera of the order Ceramiales (Rhodophyta) and were collected in six stations of the Venice Lagoon. Aglaothamnion halliae and Melanothamnus japonicas were probably introduced via shellfish products, in particular, with the American clam, Mercenaria mercenaria, and with the Japanese oyster, Magallana gigas, respectively. Polysiphonia schneideri, instead, appears to have been introduced from Spain via the Strait of Gibraltar as a fouling organism on ship hulls. The presence of closely related cryptic species in the Venice Lagoon, such as Aglaothamnion tenuissimum, Melanothamnus harveyi and Polysiphonia denudata, and the lack of diagnostic characters can lead to the underestimation of the biodiversity of these genera. For these reasons, we used DNA barcoding to discover and correctly identify these new introduced specimens.