Bathers report unusual encounter in Croatian lake

It’s not just in the Atlantic or Mediterranean that you can encounter unexpected companions while swimming. Swimmers can also be surprised by animals in lakes. An incident in Croatia shows this.

Vinkovci – Whether in the Mediterranean, the North Sea, the Baltic Sea, or even in the nearby bathing lake: spooky encounters with animals occur again and again in a wide variety of waters. Numerous bathers across Europe, as well as in German waters, are likely to have experienced this again in the summer of 2024. Sometimes strange encounters occuras two water sports enthusiasts in Croatia found out. In a lake in the east of the country, they found animals that they certainly would not have expected to see there.

Unusual encounter in Croatia – water sports enthusiasts encounter exotic animals in the sea

As they often do in the summer months, Petra and Antonio Udovčić were recently out on their surfboards on Lake Banja in Vinkovci in eastern Croatia. After they had already moved a good distance away from the shore of the lake, they discovered the strange outline of an animal in the lake water that they had previously expected to find on the beaches of the Adriatic, hundreds of kilometers away in western Croatia.

“First we met a small one, and then we were surrounded by about twenty of them,” Petra Udovčić told the Croatian online newspaper Day. What the two water sports enthusiasts saw in the lake water around their surfboards were the outlines of countless small jellyfish. The two managed to catch one of the specimens in a glass jar. They then sent it to the nearby University of Osijek.

The University’s Biology Department confirmed that the animals found in Lake Banja in Vinkovci were definitely freshwater jellyfish of the type Craspedacusta sowerbii And that they must have travelled an enormous distance to reach the lake in western Croatia.

Freshwater jellyfish in Croatian Lake originate from the Yangtze River region in East Asia

“The freshwater jellyfish originate from East Asia, from the Yangtze River area. And it is believed that their spread actually occurred through the transfer of ornamental aquatic plants or through the pet market,” said Barbara Vlaićević from the Faculty of Biology in Osijek in an interview with Day.

The small jellyfish are rarely seen. In the polyp stage, they are only about one millimeter in size, so tiny that you wouldn’t see them even from close up. When the two water sports enthusiasts discovered them in Lake Banja, the specimens there were already so big that they had already formed a hat.

The sudden presence of the jellyfish and the fact that the two water sports enthusiasts would never have suspected them in their local lakeeven frightened them at first: “At first we didn’t even expect to see jellyfish in our lake, and then we didn’t know what to expect from jellyfish. So we wondered if they could possibly harm our skin,” added the water sportswoman.

University determines jellyfish from Croatian lake – freshwater jellyfish harmless to humans

The lake’s relatively warm water suits the jellyfish. It also provides the animals with enough food. The most important thing for humans, however, is that the freshwater jellyfish cannot harm them. Although they do have tentacles, they only use them to catch their food, so-called plankton shrimp.

The tentacles cannot penetrate human skin. Touching nettles wou
ld be more painful than coming into contact with the freshwater jellyfish.

The East Asian freshwater jellyfish is already widespread in Germany

The freshwater jellyfish, which originate from East Asia, have now not only made it to Croatia, but also to German waters. The tiny and mostly unnoticed animals have been spreading here for some time now, as Herwig Stibor from the Ludwig Maximilian University Munich (LMU) on the university’s website. As a professor of aquatic ecology, his research includes the ecology of East Asian freshwater jellyfish.

“So far, only the jellyfish have been described because they are easy to see,” says Stibor. “But that is actually only a small part of the jellyfish’s life cycle. If we want to understand their invasion dynamics, we also need to understand this bottom stage, which has hardly ever been studied so far.”

In contrast to the jellyfish with its temperature requirements, polyps are extremely resilient, which makes the jellyfish a highly invasive species: “The polyps survive everything by forming permanent stages. The permanent stages are resistant to drying out, you could even freeze them at -250 degrees or boil them in sulphuric acid and they would survive,” Stibor marvels. (fh)