get to the Black Sea in two ways: the natural way, through the channels from the Mediterrean; and the human-made way, by ballast waters from all over the world. While the natural way affects the local ecosystem slowly, the human-made way sometimes triggers fast expanding distortion.
Back in the 1980s, a comb jellyfish Mnemiopsis leidyi (sea walnut) was accidentally introduced to the area. As a result, by 1989, it grew to as many as 400 jellyfish per cubic metre of water. It competed with native fish for food and also consumed their eggs and larvae, which contributed to the collapse of local fisheries. Population of sea walnuts sharply declined and the fisheries started to recover only after introduction of a biological pest control — another jellyfish species, Beroe ovate.