Salvinia molesta D. S. Mitchell (Salviniaceae) nuova per la flora d'Italia

A pentaploid (2n=45) sterile Salvinia, up to now erroneously identified as S.natans or S. auriculata, was recorded for the firts time in a canal named Fosso dell'Acqua calda and in other ones, close to Pisa (S. Giuliano Terme, Tuscany). After morphological, karyological and ecological considerations the plant was definitively referred to as S. molesta, a south american native floating fern that became widespread in tropical and subtropical aquatic ecosystems. Known as "Giant salvinia", "Kariba weed", "Aquarium watermoss" or "World's worst weed", this plant represents a significant danger in any warm, slow-moving bodies of water in all the Continets. The record is the first for Italy and possibly the only certain for Europe. The previous presence of the native S. natans in the Tuscan investigated habitats was also evidenced and briefly discussed.