Britain has experienced a remarkable increase in sightings of the frilly-mouthed jellyfish, the largest jellyfish species in UK waters, according to the Marine Conservation Society. There were 310 sightings of this species around Britain and Ireland in the 12 months to September 2025, a 230 per cent rise compared to the previous year.
Also known as the barrel jellyfish, the creature can grow to the size of a dustbin lid and has a mushroom-shaped bell with eight frilly tentacles. It is often found washed up on beaches in May and June, particularly in Scotland and Wales. Its sting is not normally harmful to humans, but experts advise against handling them as they can still sting when dead.
The reasons behind the surge are unclear, but warmer sea temperatures and changing ocean currents may be contributing factors. Jellyfish play a vital role in marine ecosystems by moving carbon through food webs and supporting biodiversity. They are also edible and could offer a more sustainable alternative to certain fish in future.
The Marine Conservation Society's annual Wildlife Sightings Report, which relies on public reports, recorded 1,327 jellyfish sightings across the UK and Ireland between October 2024 and September 2025. The most commonly spotted species was the moon jellyfish, with 316 sightings, followed by the frilly-mouthed jellyfish and the compass jellyfish.
Meanwhile, sightings of the Portuguese man o' war, a dangerous species with a painful sting, plummeted by over 80 per cent. Experts warn that even dead specimens of this species remain hazardous.
