Most people only know jellyfish from summer holidays by the sea. But in the middle of Vienna on the Danube? What sounds like one of these urban legends – such as that of the crocodile in New York’s sewage system – is reality in the Austrian capital: In the Kuchelau in northern Vienna, thousands of small jellyfish pulsate through the Danube water. Where are you from? And is that normal?
One could perhaps put it this way: it is probably part of the new normal in times of climate change. The approximately 2.5 centimeter small freshwater jellyfish of the species Craspedacusta sowerbii only develop when the water temperature is above 25 degrees Celsius for several days. Due to global warming, this is likely to happen more often in the future.
In warm water, the much more conspicuous medusae break off from the one to two millimeter small polyps, which are now found in many freshwater bodies in Austria and Germany. They consist of 99.3 percent water and thus have the highest water content ever found in an animal. “In addition to temperature, other factors are necessary for the polyps to form medusae,” says Sabine Gießler, who researches freshwater jellyfish at Munich’s Ludwig Maximilian University.
There are still a few unsolved mysteries regarding the little jellyfish
Exactly what these are is not yet known. The concentration of the greenhouse gas CO₂ could play a role, but also the phase of the moon. It is also puzzling why the polyps form medusae at all. In other cnidarians, this stage is used for sexual reproduction. But in the case of Craspedacusta sowerbii mostly only females or only males are found in one and the same body of water, so the two sexes hardly ever meet.
Unlike the jellyfish, the polyps do not swim freely in the water, but sit firmly on stones or plants or dock on boats and jetties. There are thousands upon thousands – you usually just don’t see them because they’re so tiny. But they are everywhere: Sabine Gießler looked for them in about a hundred lakes in Germany and Austria. “We found what we were looking for in 80 percent of the time,” she says.
Originally, Craspedacusta sowerbii probably comes from the Yangtze River in China, where the jellyfish is also called “peach blossom fish”. It was first discovered in Europe in 1880 – in a water lily pond in London’s Regent’s Park. The polyps probably got there along with ornamental plants from China. But they also travel with ornamental fish or in the plumage of waterfowl.
The medusae are completely harmless to humans. They have stinging cells, which they use to paralyze and kill rotifers and other small creatures, but unlike the stinging of many of their marine relatives, the stinging of the freshwater jellyfish does not penetrate human skin.
Theoretically, there is no reason not to swim in a body of water where the jellyfish are found. The medusas are even a sign of good water quality – they need clean water, so they are not found in polluted lakes and rivers.
In any case, the jellyfish from Vienna will probably disappear again soon. Unlike the hardy polyps, which can even survive the winter, medusae have a very short lifespan.