At the beginning of September 2020, freshwater jellyfish were spotted in the Upper Sûre Lake. Photographs of this jellyfish were sent by divers from the CGDIS Frogmen Group to the Water Management Administration (AGE) and the Nature and Forestry Administration (ANF). This observation made it possible to confirm the presence of the Craspedacusta sowerbii jellyfish in the Upper Sûre Lake.
It is true that the term “jellyfish” is reminiscent of marine species with a stinging nature, which can trigger painful skin reactions, similar to a burn, on direct contact with the animal. The freshwater jellyfish, on the other hand, is a harmless relative. Although it belongs to the group of cnidarians, it is completely harmless to humans.
Originally from Asia, the freshwater jellyfish has spread to other parts of the world mainly due to the introduction of aquatic plants and fish. This invasive alien species is no larger than 25 mm in diameter and prefers calm, stagnant waters that warm up considerably in summer.
As this jellyfish requires temperatures often above 20°C over a long period of time, its appearance takes place from July to October with a peak observed from the end of August to the beginning of September.
It should also be noted that these jellyfish prefer clean water. In the evening, they often come to the surface of the water and can be observed there. They can also be admired when diving at greater depths during the day. These jellyfish have also been reported in other surrounding areas. It is very likely that their proliferation will continue as the climate warms.
The problem of the appearance of blue-green algae (cyanobacteria), which is a real concern in many respects for the Upper Sûre Lake, is a phenomenon totally independent of the appearance of the freshwater jellyfish observed and described in this press release.
In 1974, the polyp of Craspedacusta sowerbii had already been discovered in Luxembourg by biologist Jacques Dahm, in the river Sûre near Wasserbillig (Dahm 1974):
Dahm, J., 1974. Die Hydrozoen Luxemburgs. Eine Bestandsaufnahme und Beschreibung der in Luxemburg lebenden Hydrozoen. Luxembourg. Mém. sci. asp.-prof. 232 p.