Ten alien species were recorded in a first-time survey of the benthic biodiversity of the upper sublittoral at Sigri Bay (Lesvos Island, north-eastern Aegean Sea). The record of the mollusc Syrnola fasciata is the first in Hellenic territorial waters. The records of the brown alga Stypopodium schimperi, the bivalve Septifer cumingii, the gastropods Syrnola fasciata and Smaragdia souverbiana and the bony fish Siganus luridus constitute evidence for a further northwards extension of their geographic ranges in the Aegean Sea. The record of Cerithium scabridum is the first in Lesvos Island and partially fills the gap in the distribution of the species in the eastern Aegean Sea. We also recorded the green alga Caulerpa cylindracea, the angiosperm Halophila stipulacea and the bivalve Pinctada imbricata radiata, which are widely distributed in the eastern Mediterranean. The polychaete Marphysa adenensis, which was also found at Sigri Bay, has recently been reported for the first time from the Mediterranean as a cryptogenic species.

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.