How to identify UK jellyfish

Jellyfish occur throughout UK seas with large blooms of most species appearing in the spring and lasting through to autumn. The barrel jellyfish can be spotted in our seas all year round.

Help us learn more about jellyfish

Thousands of people have shared sightings of jellyfish from around the UK, helping to build an extensive data set of six jellyfish and two jellyfish-like hydrozoan species.

Understanding trends in jellyfish distribution and numbers can help us understand where leatherback turtle feeding grounds might be, as well as potentially indicating the impacts of climate change on our ocean.

If you have any photos to submit, please email them to [email protected] or tag us on social media with the hashtag #jellyfishsighting.

Report a jellyfish sighting

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Barrel jellyfish (Rhizostoma octopus)

Up to 1m in diameter. Robust with a spherical, solid rubbery bell, which can be white or pale pink, blue or yellow and fringed with purple markings. The bell lacks tentacles but eight thick, frilled arms hang from the manubrium. Mild sting.

Barrel jellyfish.jpg

Barrel jellyfish

Blue jellyfish (Cyanea lamarckii)

Up to 30cm, similar shape to the Lion's mane jellyfish but smaller with a blue bell through which radial lines can be seen. Confusingly, a yellow colour variant also occurs in UK waters. Mild sting.

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Blue jellyfish

By-the-wind-sailor (Velella velella)

Not a jellyfish but a floating, solitary hydranth. Up to 10cm long and blue-purple in colour. Upright sail and chitinous float are diagnostic, with a mass of small tentacles surrounding the mouth on the underside. Occurs in vast swarms.

By the wind sailor.jpg


Compass jellyfish (Chrysaora hysoscella)

Typically up to 30cm. Colour variable, but usually has pale umbrella-shaped bell with diagnostic brownish V-shaped markings, 32 marginal lobes and 24 long, thin tentacles, four long, thick, frilled arms hang from the manubrium.

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Compass jellyfish

Lion's mane jellyfish (Cyanea capillata)

This jellyfish stings. Large, usually 50cm but can reach 2m in diameter. Large reddish brown, umbrella-shaped bell with a mass of long, thin hair-like tentacles as well as short, thick, frilled and folded arms.

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Lion's mane jellyfish

Mauve stinger jellyfish (Pelagica noctiluca)

This jellyfish stings. Up to 10cm. Has a deep bell with pink or mauve warts, 16 marginal lobes and eight marginal, hair-like tentacles. Manubrium bears four longer frilled arms with tiny pink spots.

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Mauve stinger jellyfish

Moon jellyfish (Aurelia aurita)

Up to 40cm in diameter. Transparent, umbrella-shaped bell edged with short hairlike tentacles. Recognised by the four distinct pale purple gonad rings in the bell. Manubrium (mouth and arms, underside and centre of bell) bears four short, frilled arms. Mild sting.

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Moon jellyfish

Portuguese man-of-war (Physalia physalis)

This animal stings. Not a jellyfish, but a floating colony of hydrozoans. The oval-shaped, transparent float with crest is characteristic. Blue-purple in colour, with many hanging ‘fishing polyps’ below that may be tens of metres long. Extremely dangerous to humans due to their powerful sting. Rare in the UK but if found in numbers should be reported to the local authorities.

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Portuguese man-of-war

Staying safe and respecting our wildlife

While most UK jellyfish are harmless others have a painful sting. For your own safety, it's best not to touch them.

What to do if you get stung by a jellyfish

If you do get stung, while swimming in the ocean or walking on the beach (even a dead jellyfish can sting), here's what you should do:

  • Don't panic. Remember that most jellyfish stings are not emergencies.
  • Get out of the water as soon as possible.
  • Tell a lifeguard if there is one on duty, so they can warn other bathers.
  • Follow the NHS guidance on treating a jellyfish sting.

How we use your data

We use the data to tell us what is happening with jellyfish populations around the UK – specifically when and where they are occurring.

We publish this data with our university partners and this gives us baseline knowledge that we can use to compare any changes that might happen in the future as a result of major environmental issues such as climate change. You can read our first paper from the survey data here.