Some unexpected visitors have arrived in the Austrian capital this summer.

The tranquil waters of the Danube River have become home to hundreds of jellyfish, sparking both fascination and concern among locals. 

Journalist Nadja Hahn from Austrian radio station Ö1 was the first to bring attention to this intriguing sight when she tweeted a photo of a “carpet” of jellyfish on Sunday.

Her tweet raised questions among the inhabitants of the Danube city, prompting them to wonder whether this was a normal occurrence or cause for concern.

Ich rudere seit 17 Jahren in der Kuchelau in Wien. Quallen haben wir dort noch nie gesehen. Was hat das zu bedeuten? Ist das normal? Was sagen die Wissenschaftler unter euch? pic.twitter.com/w0hELkxk18

— Nadja Hahn (@nadjasnews) July 23, 2023

Experts, however, have assured the public that the aquatic visitors aren’t dangerous.

Unlike their saltwater cousins, freshwater jellyfish have weak stinging cells that can’t penetrate human skin, making them harmless to humans.

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Dr. Daniel Abed-Navandi – deputy director at the House of the Sea in Vienna – even encourages residents to embrace the presence of these creatures and to get up close with them in the water.

“These jellyfish only live for one or two weeks, and unless you vigorously disturb them, you won’t harm them. You should definitely not miss the chance to see the medusae.”

One visitor to the nudist beach at the Kuchelau Harbour in Vienna’s Döbling district recently told the Heute newspaper that she had swum with these creatures herself.

According to Gerald Loew, head of the responsible Municipal Department 45 for Vienna’s waters, the presence of jellyfish in the Danube is actually a positive sign.

“The animals are an indication of high water quality,” Loew told the Kurier newspaper.

Where did they come from?

The fact that these animals have been seen from time to time in recent years – most recently in 2020 and 2021 – is related to the rising water temperatures caused by the recent heatwaves. Besides good water quality, the jellyfish need temperatures which are consistently above 25 degrees Celsius.

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Interestingly, while jellyfish are commonly associated with marine environments, some species of freshwater jellyfish have made their way to Central Europe. These exotic species are believed to have been introduced from Asia and have now settled in the Danube. Researchers were surprised to find freshwater jellyfish appearing in lakes in Bavaria last year, further confirming their presence in the region.

For most of their lives, the creatures exist as tiny polyps, which are only about two millimeters in size and anchored to the bottom of the water bodies. Once the water temperatures surpass 25 degrees Celsius, they transform into the familiar medusae form and rise to the water surface to feed on sunny days.

Will they go away?

The invasion of jellyfish is likely to become a regular feature of Vienna’s waterways, as Gerald Loew explained that, once these creatures establish themselves in an environment, they are there to stay. For water sports enthusiasts and swimmers, this means embracing the presence of these harmless creatures in the city’s riverways.