Freshwater jellyfish appear in Bavaria's lakes: immigrants from China are a mystery to researchers

Created: 2022-09-02 20:32

By: Cornelia Schramm

Such freshwater jellyfish also float pulsating through Bavarian lakes – almost transparent and only the size of a coin.

Anyone who sees one can report it to the LMU for research purposes at jellyfish@bio.lmu.de.

© LMU

There are jellyfish in Rimini, but not in the Isar or in the crystal-clear mountain lake – one would assume.

But an immigrant from China is now being sighted more and more often in Bavaria: the freshwater jellyfish.

In contrast to the stinging jellyfish, it is harmless - but has been a mystery to researchers for quite some time

Planegg/Martinsried – It is only the size of a two-euro coin and still a mystery to science: the freshwater jellyfish “Craspedacusta sowerbii”.

It is actually at home in far-off Asia, in the Yangtze River in China - and yet the wobbly, white animals are now being spotted more and more often by bathers in rivers and lakes in Bavaria.

In warm summers like this, there are more of them, researchers suspect.

China jellyfish appeared in Bavaria's lakes: a biologist explains the phenomenon

Sabine Gießler is fascinated by the immigrants from China.

The 66-year-old teaches aquatic ecology at the Ludwig-Maximilians-University.

Dozens of freshwater jellyfish swim in an aquarium in the laboratory of the Faculty of Biology in Planegg near Munich.

"In this form, the jellyfish are of course very eye-catching for swimmers and divers," she says.

"But this stage is actually only a short part of their life cycle." Craspedacusta sowerbii - these are also thousands upon thousands of tiny polyps of one to two millimeters in size that "dock" in the water on stones, wood, plants or even on boats or jetties. , according to Giessler.

Only for sexual reproduction do they pinch off male and female "medusae", which develop into conspicuous shoal animals that we can see in the water with the naked eye.

"Unfortunately, the jellyfish only survive five to six weeks in the laboratory," says Gießler.

"These animals, which can be up to two centimeters in size, live longer in nature." However, they can only be found from summer to autumn.

The tiny polyps, which can only be seen under a microscope, survive the winter.

"The cnidarians are highly developed," says Gießler.

There are numerous other animals in Bavaria's lakes.

From crayfish to eel: These are the typical residents of Bavarian bathing lakes.

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Bavaria: Jellyfish dive up at night to look for food

“In their sexual form, they swim pulsating through the water and react to light stimuli.

During the day, most jellyfish swim far down in the lake to protect their eggs and sperm from UV light, and at night they dive up to follow their food.” Gießler has researched water fleas for a long time.

"They are the jellyfish's food." And because the freshwater jellyfish competes so much with fish and other local predators, it affects the food web in the lake.

The jellyfish polyps also eat zooplankton.

“Like rotifers and mini worms.

They slurp the latter like spaghetti until they almost burst.” The fact that it is getting warmer due to climate change favors their spread.

"Swarms of up to 1,000 animals have already been sighted in Austria, which looked like a sea of ​​soap bubbles," says Gießler.

In the Inn, in the Danube, in the Isar, students collected stones and snorkeled for jellyfish from the Haager Weiher near Freising to the Weicheringer See.

"We sampled over 100 bodies of water - and found polyps in 70 percent of the cases," she says.

“These are just random samples, as the animals are difficult to find.

But I assume that they exist in almost every body of water.

After all, polyps are also found in the plumage of waterfowl or on boats and are thus transported from sea to sea.”

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Jellyfish from Asia probably came to Europe through trade

Munich researchers sighted the freshwater jellyfish in the Old Botanical Garden as early as 1905.

It was first discovered in London in 1880.

At the turn of the century, trade with Asia was booming.

Gardens with lily ponds and aquariums were modern.

"It is assumed that the polyps were brought in when trading aquatic plants," says Gießler.

Nobody recognized the stowaway.

To this day, there are still many questions: what does it depend on for polyps to form jellyfish?

How does the species spread?

Does it harm the local ecosystem?

What predators does she have?

But one thing is certain: “The freshwater jellyfish is completely harmless to humans.

Their nettle cells cannot penetrate our skin,” says Gießler.

"The jellyfish are also a sign of very good, clean water quality."