The biota of the Mediterranean and the Black Sea has started to change significantly over the last few decades, due to the introduction of non-indigenous species as a result of Lessepsian migration, ship ballast water, range expansion of Atlantic species, intentional or unintentional introduction and climate change.
The Black Sea has suffered from the non-indigenous invasive comb jelly Mnemiopsis leidyi since the late 1980s, which has led to biodiversity loss and a decrease in fish catch, mainly driven by a sudden collapse of small pelagic species, in particular of the European anchovy. The trend of Mediterranization has also contributed to changes in Black Sea biota, as many species that used to be restricted to the Mediterranean have infiltrated the Black Sea, presumably due to climate change. Rapa whelk (Rapana venosa), a sea snail, is the most harvested non-indigenous invasive species by Black Sea countries and has also been commercially exploited in the Marmara Sea. The Marmara Sea, situated between the Black Sea and the Aegean Sea, plays a crucial role in the range shifts of invasive species due to its exchange of water mass and marine biota. Like the Black Sea, the Marmara Sea has also been negatively impacted by the comb jelly M. leidyi, showing a recovery of small pelagic fish species only in recent years. Lessepsian fishes and invertebrates are increasingly found in the Marmara Sea, but overall impacts are not yet known.
Numerous Lessepsian species have invaded the eastern Mediterranean Sea entering through the Suez Canal, while others have expanded through the central and western sectors of the basin. A number of fish and invertebrate species originating in the Atlantic have also reached the eastern part of the Mediterranean Sea. In total, over 900 non-indigenous species have been reported in the Mediterranean Sea, and almost 300 non-indigenous species in the Black Sea. Over half have established permanent populations and are geographically expanding.