Scientists present first comprehensive inventory of Mnemiopsis leidyi in Europe. When the American comb jelly Mnemiopsis leidyi , also known as sea walnut, conquered the Black Sea as a new habitat 35 years ago, the ecosystem there changed sustainably. The economically important anchovy stocks collapsed since the fish had to compete with the jellyfish for food. Against this background, scientists, fisheries associations and environmental authorities were alarmed when, in , the sea walnut also appeared in northern European waters. Although effects similar to those in the Black Sea have not been observed in the Baltic and in the North Sea yet, scientists are still closely monitoring the development - particularly since many questions on invasion pathways are still largely unclear.