Non-indigenous species are those organisms that are not found in their natural habitat but were brought intentionally or accidentally to a new area. They have a major impact on biodiversity, ecosystems, economy and human health. The most common vectors for the spread of aquatic alien species are shipping and aquaculture. Mnemiopsis leidyi is a cthenophore introduced by ballast waters from American continent in Eurasian seas, where it settled and caused great environmental and economic damage. It was first recorded in the 1980s in the Black Sea, where after a few years it destroyed fisheries and caused great monetary damage. It further spread to the Aegean, Azov, Marmara, Caspian, Mediterranean and Adriatic Seas. It has also been recorded in the Baltic and North Seas. This ctenophore has a number of abilities that enable it to survive and expand its population. Some of them are: hermaphroditism, which allows all individuals to produce fertile larvae; high fecundity; rapid growth; nutritional flexibility and high physiological tolerance to temperature, salinity and dissolved oxygen.