By far the most common jellyfish species in our waters in the Baltic and North Sea is the moon jellyfish (Aurelia aurita). It appears mainly in summer and early autumn and in some years can be seen in large swarms, known as jellyfish blooms.
The moon jellyfish is characterised by four ring-shaped sex organs in the centre of its transparent bell. They resemble small ears and gave the species its German name. The bell is around 5 to 40 cm across, and the tentacles can range from a few centimetres to about a metre in length.
For humans, the moon jellyfish is considered largely harmless because its stinging cells usually cause no more than mild skin irritation.
However, the lion’s mane jellyfish (Cyanea capillata) also lives in the Baltic and North Sea. It is also known as the yellow stinging jellyfish and is usually referred to by coastal residents as a "fire jellyfish". With its long tentacles it can cause painful stings. Even detached tentacles can continue to sting for some time.
Compared with the moon jellyfish, the fire jellyfish is significantly larger, with a bell diameter of about 20 to 50 cm, and tentacles that can reach 10 to 30 metres in length.
Fire jellyfish are much more common in the North Sea than in the Baltic. They favour cooler, saltier water. They are regularly sighted along the coasts of Sylt, Amrum, Norderney and the East Frisian Islands, as well as in the North Atlantic and off the coasts of the United Kingdom, Norway and Iceland. They are also found off Spain, France, Italy, Greece, Croatia and Turkey.
Jellyfish species in the Mediterranean, some friendlier than others
Holidaymakers swimming in the Mediterranean encounter other stinging jellyfish species much more frequently.
The mauve stinger (Pelagia noctiluca) is considered one of the most unpleasant jellyfish species for bathers in the Mediterranean. It occurs mainly in the western Mediterranean, for example off the coasts of Spain, France and Italy, but also in the Adriatic (including off Croatia and Montenegro), in the Aegean, in parts of the eastern Mediterranean and off the North African coast.
This striking jellyfish is usually violet, pink or reddish in colour and reaches a bell diameter of around five to 15 centimetres. Its long, thin tentacles can extend for several metres. Contact can cause burning pain, red patches on the skin, wheals and persistent itching. People with sensitive skin may experience more severe reactions.
The mauve stinger owes its name to its ability to produce faint bioluminescence: in darkness it can emit a soft glow.
The compass jellyfish (Chrysaora hysoscella) is another conspicuous jellyfish species in the Mediterranean. It has a yellowish-brown bell marked with characteristic lines that resemble a compass rose, as well as long tentacles. Typical holiday regions where the compass jellyfish is found include the coasts of Spain, Italy, Greece, Turkey and the Adriatic.
Contact with its tentacles can be painful, although symptoms are generally less severe than with the mauve stinger.
The barrel jellyfish (Rhizostoma pulmo) also occurs in the Mediterranean and is among the largest jellyfish species there. Its bell can reach an impressive diameter of up to around 60 centimetres. Despite its considerable size, it is usually not very dangerous to humans because its stinging cells are relatively weak. Contact often causes no symptoms or only mild discomfort.
Because of its imposing appearance and harmless nature, it is considered a "gentle giant" among Mediterranean jellyfish.
What to do if you are stung by a jellyfish?
If you are injured by a jellyfish, you should not rub or scratch the affected area, as this can activate additional stinging cells. The skin should be gently rinsed with seawater and any visible tentacles carefully removed with tweezers or a plastic card.
Freshwater should be avoided in the case of jellyfish injuries, as it can trigger stinging cells that have not yet discharged. Because of the difference in salt content between freshwater and seawater, the cells undergo osmotic changes and can release more venom into the skin. This may intensify the pain and worsen the skin reaction.
To relieve pain, the affected area can be immersed in warm, but not scalding, water (around 40-45 °C) for 20 to 45 minutes or treated with warm compresses.
Alcohol and urine should also be avoided, as they could aggravate the symptoms. If there is shortness of breath, swelling, severe pain, dizziness or circulatory problems or other serious symptoms, medical attention should be sought immediately.
There are jellyfish in Lake Constance too
There are no marine jellyfish in Lake Constance or other local inland waters, but the freshwater jellyfish (Craspedacusta sowerbii) does occasionally appear there, a small species that originally comes from East Asia and has now spread to freshwater habitats worldwide. The jellyfish lives mainly in slow-flowing or still waters whose shallow shoreline areas can warm up considerably.
It usually appears in late summer, reaches a diameter of only one to three centimetres and is considered harmless to humans because its stinging cells generally cannot penetrate human skin. Its diameter is up to 2.5 centimetres.
In an unusual incident in August 2025, the Gravelines nuclear power plant in northern France had to be completely shut down for a time when a massive swarm of jellyfish clogged the filters of the cooling-water pumps. Four reactors were automatically taken offline; the other two were already out of service for maintenance.
Back in 2013, a swarm of jellyfish temporarily put a reactor at the Oskarshamn nuclear power plant on the Kalmar Sound on Sweden’s east coast out of action after the animals blocked the cooling system. Similar incidents have also occurred in Japan, for example at the Hamaoka nuclear power plant in 2006.
According to the operators, there was never any danger to staff, the public or the environment. Only the jellyfish themselves were at risk: the pressure in the filter system can be fatal for them.
This text was translated with the help of artificial intelligence. Report a problem : [feedback-articles-en@euronews.com].
