A boat owner reported seeing “millions” in Cardigan Bay in June. An influx of “alien” ones were reported in Torquey last month and, last week, a swarm entered the filter drums of the pumping stations at France’s largest nuclear power plant, shutting down four reactors. Why are there so many jellyfish around this year?

The number of UK jellyfish sightings recorded by the Marine Conversation Society rose by 32% last year, compared to the previous 12 months and a greater number still are likely to be recorded this year. That’s because jellyfish blooms follow currents of warm water. Warm sea surface temperatures. Record-high sea surface temperatures, recorded by the Met office this spring, can also trigger jellyfish to reproduce more rapidly. “All jellyfish can sting, even dead jellyfish, but the severity varies depending on the species,” says Citizen Science Programme Developer at the Marine Conservation Society, Anna Bunney. So what do you need to know?

How to spot the worst stingers

In the UK, the most commonly reported species has been the moon jellyfish, says Anna: “these are recognised by the four distinct pale purple rings on their transparent bell.”

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Compass jellyfish were also widely spotted she adds: “these usually have a pale umbrella-shaped bell with brownish V-shaped markings (hence the name) and they have 24 long, thin tentacles and four long, thick, frilled arms.”

Both of these have a mild sting, so if you spot one, do it’s best to give them a wide berth. One you really want to watch for, however is the lion’s mane jellyfish pack. “These pack a painful sting,”: says Anna. “They are large, reddish brown in colour and have a mass of long, thin hair-like tentacles as well as short, thick, frilled and folded arms.”

Then there’s the scariest stinger of all. When the Marine Conservation Society released its annual Wildlife Sightings report, mark World Jellyfish Day on 3 November last year, the number of man o’ war sightings has increased fourfold increase from the 65 sightings logged in the 2022-2023 report. “Portuguese man o’war have the most painful sting in our seas and are becoming more frequent, being the third most reported last year,” says Anna. How to spot them? “They have a transparent, gas filled float with a crest, are blue-purple in colour, and have many hanging ‘fishing polyps’ below that may be tens of metres long.”

What to do if you’re stung

First things first, “don’t panic,” says Anna: “most jellyfish stings are not emergencies. If you’re swimming, get out of the water as soon as you can and tell a lifeguard (if there’s one on duty), so they can warn others in the water.” Next up, rinse the affected area with seawater (not fresh water) and try to remove any spines from the skin using tweezers or the edge of a bank card (not your bare hands). Then, soak the area in very warm water (as hot as can be tolerated) for at least 30 minutes and finally take a painkiller.

You should seek medical advice if severe pain won’t go away, or your high temperature or wound doesn’t get better. You’ll need urgent help if you have chest pains, difficulty breathing, vomiting, fits, severe bleeding or swelling or loss of consciousness.

The good news is that you DON’T need to pee on it. It won’t help. Nor should you apply ice or cover the wound.

What about the ones washed up on the beach?

If you spot a jellyfish on the beach, it’s likely that is already dead, says Anna: “Jellyfish aren’t very strong swimmers and are at the whim of the waves and ocean currents, meaning they are often blown off course. If you spot one washed up, or in the sea, please report it to the Marine Conservation Society at www.mcsuk.org/sightings, as it helps us to understand changes in our seas, like warming waters. Do not pick them up and return to the water, as remember – even dead jellyfish can sting!”

Are they dangerous to dogs?

“Just like humans, jellyfish stings can be painful for your dog,” says Anna. “We advise keeping your dog on a lead if you see jellyfish stranded on the beach, and if your dog does get stung, consult your vet as soon as possible.”

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