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Dugesia (Girardia) tigrina is a Nearctic freshwater triclad native to North America, now amply distributed in most European countries as the result of a reportedly unique introduction into north-west Europe early this century and its dispersal southwards and eastwards in the last decades. All European populations described so far are externally spotted, with a pigmented pharynx, and, apart from two sexual populations recently described in England, they are fissiparous. This uniformity contrasts with the variability reported in its native area, thus supporting the hypothesis of a single introduction. However, thorough morphological, karyological and biochemical comparative studies of European populations have never been properly done.

We have undertaken such a study comparing 24 populations present in a restricted area of the western Mediterranean. The results show an unexpectedly ample variation in external and pharyngeal pigmentation patterns, the presence of sexually reproducing populations, and ample variation in their biochemical patterns as revealed by isoelectric-focusing. This is an indication that European Dugesia (G.) tigrina is far more diverse than previously thought, suggesting that several, independent introductions from its native area have recently occurred, and the implications of these findings in biogeographical and taxonomic terms are discussed.