Mnemiopsis leidyi was introduced in the Black Sea in the 1980s, where only one species of comb jelly, the small sea gooseberry Pleurobrachia pileus occurred until then. The most likely cause of its introduction is accidentally by merchant ships' ballast water. The first Black Sea record was in 1982.[5]
By 1989, the Black Sea population had reached the highest level, with some 400 specimens per m³ of water (>10 animals/cubic foot) in optimal conditions.[3] Afterwards, due to depletion of foodstocks resulting in lower carrying capacity, the population dropped somewhat. -- Wikipedia