The Sea Walnut on a Campaign of Conquest

Glassy and ethereally delicate – Mnemiopsis leidyi, or more popularly the warty comb jelly or sea walnut, looks harmless. Yet appearances are deceiving: At the beginning of the 1980s, the comb jellyfish travelled from the American West Coast as far as to the Black and Caspian Sea in the ballast water tanks of ships. Four years ago, it was also discovered in the Baltic and North Sea. The jellyfish arrived as a voracious conqueror and diminishes the indigenous fish stock. “Fish eggs and larvae are at the top of its bill of fare. Furthermore, it devours zooplankton and thus takes away food from the fish”, says biologist Professor Dr. Karen Wiltshire. The researcher is Director of the Biological Institute Helgoland (BAH) and is Deputy Director of the Alfred Wegener Institute in Bremerhaven. Together with her colleagues she researches how the stranger behaves in its new habitat and why it spreads with such enormity. For this, the jellies are for the first time caught in a targeted manner and observed in the laboratory. The sea walnut is comparatively robust and survives being caught rather well, yet it still is tricky to simulate the natural sea environment in an aquarium. For instance, turbulences support the jellies in swimming. It is difficult creating just sufficient turbulence to maintain the sea walnut in abeyance, yet little enough not to flush it over the rim.

Observation shows that the jellies are masters of conquest: They can reproduce after only two weeks and in addition they can cope with great differences in salt content and temperature. In their actual tropical home they live in water temperatures of above 25 degrees, here they live with temperatures below 10 degrees even – they only stop at less than 2 degrees. “Previously, the temperature in winter dropped even more. This was the case also this year, but it happens more and more rarely. This, of course, is much in favour of the jelly”, says Wiltshire.

In general, jellies appear more and more frequently in the North Sea – amongst them also predators: The lobed comb jelly (Bolinopsis infundibulum) and the melon jelly (Beroe gracilis), for instance, focus on relatives. Perhaps they keep the sea walnut at bay. It worked in the Black Sea, there, too, a natural enemy appeared.