The native habitat of M. leidyi includes estuaries and coastal regions along the eastern coast of North and South America (GESAMP, 1997). Mnemiopsis leidyi is polymorphic, with wide environmental tolerance; therefore, environmental conditions in the southern Eurasian seas seem suitable for the species’ establishment in this area. The spread of M. leidyi is of particular concern since the species strongly affects all levels of most invaded ecosystems, including the fisheries in the Black, Azov and Caspian Seas, although effects were less pronounced in the Aegean Sea (Shiganova et al., 2001; Shiganova et al., 2004a, b, c).