The comb jellyfish arrived on ships from the American Atlantic coast in 1982. It eats both zooplankton, the food of commercially important fish in the Black Sea, and the eggs and larvae of the same fish species.
With no enemies in their new home, the jellies propagated at an alarming rate. By the mid-1990s, they accounted for 90% of the total biomass in the Black Sea - a biomass more than the total annual fish catch around the world. The species quickly spread into the neighbouring Azov Sea too.
The invasion contributed to the near collapse of Black Sea commercial fisheries within a few years. The once quite prosperous seafood industry has lost about US$1 billion since the jellies were released. Anchovy fisheries in the Azov Sea, already under stress from pollution
, have completely collapsed.
Dolphin numbers in the Black and Azov Seas also dropped dramatically, as the fish they used to feed on disappeared. The entire ecosystem has been disrupted - the jellies have even reduced the amount of oxygen in the Black Sea.
They've now entered the Caspian Sea, where they are similarly wreaking havoc, and have been found in the Baltic Sea and along the Atlantic coast of Norway too.