The presence of the pantropical ascidian Microcosmus exasperatus has been an object of some taxonomic confusion in the Mediterranean Sea. A closely related species, M. squamiger was previously reported in the north of Tunisia (in 1962) as M. exasperatus; it has spread to the western Mediterranean and the northern part of the Ionian Sea, whereas, the reliable report of M. exasperatus has been made more recently (in 1998) in the Gulf of Gabès (South Tunisia), and later studies have confirmed its presence in the eastern Mediterranean basin (Lebanon and Israel). The occurrence of this species in Izmir (Turkey) extends its distribution range to the Aegean Sea. At present, the distributions of both species of Microcosmus are not overlapping in the Mediterranean, suggesting a different route of entrance, probably by the maritime traffic (M. squamiger via the Strait of Gibraltar; and M. exasperatus via the Suez Canal). The discontinuity in the distribution of M. exasperatus in the Mediterranean is analysed and discussed.
