1 - Introductions of exotic macroalgae have increased recently on European shores. Each introductioninvolves at least one vector of transfer. For macroalgae, the potential vectors are aquaculture(intentional or accidental introduction), fouling on hulls, ballast water, aquarium trading, fishingnets.2 - Coastal lagoons, including Venice and Thau Lagoon, developed into major hotspots of marinemacrophyte introductions in the Mediterranean Sea. Moreover, in shallow lagoons and estuaries,eutrophication processes have resulted in the development of macroalgal biomass.3 - The most characteristic species of these macroalgal communities include members of the ulvophyceangenus Ulva L. In foliose Ulvales, simple morphology and anatomy, rampant convergence, remarkabledegrees of phenotypic plasticity in response to environmental factors tend to confound attempts atidentification and make cryptic introduction difficult to detect.4 - During a survey of macroalgal biodiversity in Venice Lagoon, among the pool of exotic speciesfound, there was an Ulva differing from the Atlantic and Mediterranean species in both vegetativeand reproductive features. Detailed observations consented us to identify the taxon as Ulva pertusaKjellman, previously reported in the Mediterranean only for Thau Lagoon
