Mnemiopsis - As An Invasive Species

As An Invasive Species

1980s – Black Sea

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.

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. Afterwards, due to depletion of foodstocks resulting in lower carrying capacity, the population dropped somewhat.

In the Black Sea, M. leidyi eats eggs and larvae of pelagic fish. It caused a dramatic drop in fish populations, notably the commercially important anchovy Engraulis encrasicholus (known locally as hamsi, hamsiya, hamsa, etc), by competing for the same food sources and eating the young and eggs. Biological control was tried with Beroe ovata, another comb jelly, with some degree of success; it appears as if a fairly stable predator-prey dynamic has been reached.

1999 – Caspian Sea

In 1999 the species was introduced in the Caspian Sea. The result was that 75% of the zooplankton was depleted, thereby affecting the entire food chain of the lake.

2006 – North and Baltic Seas

Since then, the species has apparently spread throughout the Mediterranean basin and the northwestern Atlantic. In 2006, it was first recorded in the North Sea, and since October 17, 2006 in the western Baltic Sea, namely the Kiel Fjord and The Belts. Up to 100 animals/m³ (c. 3/cubic ft) were counted in the Baltic, whereas the population density in the North Sea was at a much lower 4 animals/m³ (about 1 animal/9 cubic ft) at most.

One year later, the Baltic population of M. leidyi was found to have spread east to the Gotland Basin and the Bay of Puck. The impact of the species on the already heavily stressed Baltic ecosystem is unknown. The species overwinters in the deep waters where temperature does not drop below 4 °C (39 °F); the fact that the Baltic is heavily stratified, with the waters above and below the halocline mixing little, is believed to aid its survival.

Apart from the widespread P. pileus, 3 comb jelly species are occasionally drifted into the Baltic from the North Sea but do not seem to be present as a stable population of significant size: Bolinopsis infundibulum, Beroe cucumis and Beroe gracilis. The second species might potentially be used for biological control.

The route of dispersal of M. leidyi to the North Sea/Baltic region is unknown. It might have occurred naturally by drifting individuals, or with ballast water of ships, either from its natural range or from the Black Sea, via the Mediterranean and eastern Atlantic. At least technically possible given the species' euryhaline habits is an alternate route of dispersal through continental Europe, being carried with ballast water in ships travelling from the Black Sea to the Rhine Estuary via the Rhine-Main-Danube Canal. The latter route is known to be the point of entry into continental Europe for numerous invasive freshwater neozoons from the Ponto-Caspian region, such as the zebra mussel, the quagga mussel, the amphipods Dikerogammarus villosus and Chelicorophium curvispinum, and the polychaete Hypania invalida.