Numerous reports of poisonous jellyfish
It is only a few centimetres in size and has a poisonous sting. This summer, over a hundred reports of the clinging jellyfish have been received.
‘The clinging jellyfish has spread to several locations along the Swedish west coast and seems to be here to stay,’ says Doris Björling, jellyfish researcher at the University of Gothenburg.
During the hot summer of 2018, several sightings of clinging jellyfish were reported on the Swedish west coast. Since then, reports have been few and far between, concentrated in a limited area within Tjörn and Orust. However, during the summer of 2025, many reports have been submitted to Artportalen. Over a hundred observations have been made, from Strömstad in the north to Kungsbacka in the south.
‘The species benefits from persistently warmer water temperatures and then forms free-swimming jellyfish. It seems that conditions have been unusually favourable this year,’ says Doris Björling.
Probably two species
The clinging jellyfish originally comes from the western Pacific Ocean. There are many theories about how the species came to Sweden, for example with imported oysters, in ballast water or on seaweed that came with boats, but no one knows for sure.
‘What we are sure of is that it spreads with the help of humans. When we look at maps of findings around the world, they are scattered along the coasts, which is a sign that they did not get there on their own‘, says Doris Björling.
The first discovery of clinging jellyfish in Swedish waters was made as early as 1923. However, there are no reports that the jellyfish found at that time were poisinous. However, the jellyfish reported since 2018 stings. This leads researchers to suspect that they are different species.
‘It is probably two different species that are similar in appearance, one of which stings and the other does not. This is something that researchers are currently investigating,’ says Doris Björling.
When to seek medical attention
Although the clinging jellyfish is small, only up to a couple of centimetres in diameter, it has a powerful venom. Symptoms vary from person to person and are not dangerous in most cases, but in cases of extensive exposure and in sensitive individuals, they can lead to serious symptoms such as breathing problems, hallucinations and fever. This summer, several people have sought medical attention after being stung by clinging jellyfish.
‘If you only feel a burning sensation in one place, like a normal jellyfish sting, you don't need to do anything. If you are stung in several places, experience other symptoms or feel concerned, you should contact medical services,’ says Doris Björling.
How can I avoid clinging jellyfish when swimming?
‘Swim in deep water or on clean sandy bottoms; jellyfish thrive in shallow bays with eelgrass or seaweed‘.
Text: Karl-Johan Nylén
The clinging jellyfish, Gonionemus vertens, is transparent and measures two to three centimetres in size, not including its tentacles. It can be difficult to spot, but can be recognised by a red, orange, brown or yellow cross across its bell. The species originates from the northern and western parts of the Pacific Ocean but has spread to the Atlantic and is now found in most of Europe's sea areas, except for the Baltic Sea.