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.

Learning more about jellyfish

Jellyfish are important for marine ecosystems. They play a huge role in lots of marine food chains. They even provide habitats for some smaller species, who shelter under jellyfish bells, protected from predators by their tentacles. Jellyfish are also a great indicator of change in our oceans, so tracking their numbers each year can help us to spot changes in climate, fishing activity or ecosystems.

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UK jellyfish

Thousands of people have shared sightings of jellyfish from around the UK, helping to build a huge data set of six jellyfish and two jellyfish-like hydrozoan species. Hydrozoans can be either solitary individual animals, or, like the two species we are recording, can live in colonies, where groups of microscopic individuals live and work together, each with a different role to play including, feeding, reproducing and protecting.

Here are some top tips and identifying features to help you decide which species you have spotted.

Barrel jellyfish (Rhizostoma pulmo)

Barrel jellyfish grow up to 1m in diameter. They're robust with a spherical, solid rubbery bell, which can be white or pale pink, blue or yellow and fringed with purple markings. Their bell lacks tentacles but eight thick, frilled arms hang from the manubrium. Careful! They have a mild sting.

Barrel jellyfish.jpg

Barrel jellyfish

Blue jellyfish (Cyanea lamarckii)

Blue jellyfish grow up to 30cm. They're a similar shape to the Lion's mane jellyfish, but are smaller and have a blue bell through which radial lines can be seen. Confusingly, a yellow colour variant also occurs in UK waters. Careful! They have a mild sting.

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Blue jellyfish

By-the-wind-sailor (Velella velella)

This isn't a jellyfish but a floating, solitary hydranth! By-the-wind-sailors grow up to 10cm long and are blue-purple in colour. They have an upright sail and the chitinous float is diagnostic, with a mass of small tentacles surrounding the mouth on the underside. Look out! They occur in vast swarms.

By the wind sailor.jpg


Compass jellyfish (Chrysaora hysoscella)

Compass jellyfish are typically up to 30cm. The colour varies, but they usually have a pale umbrella-shaped bell with diagnostic brownish V-shaped markings. They have 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. Lion's mane are large - they're usually 50cm, but can reach 2m in diameter! They have a large reddish brown, umbrella-shaped bell with a mass of long, thin hair-like tentacles as well as short, thick, frilled and folded arms. Be very careful!

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Lion's mane jellyfish

Mauve stinger jellyfish (Pelagica noctiluca)

This jellyfish stings. Mauve stingers are up to 10cm. They have a deep bell with pink or mauve warts, 16 marginal lobes and eight marginal, hair-like tentacles. The manubrium bears four longer frilled arms with tiny pink spots. Be very careful!

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Mauve stinger jellyfish

Moon jellyfish (Aurelia aurita)

Moon jellyfish grow up to 40cm in diameter. They have a transparent, umbrella-shaped bell edged with short hair-like tentacles. They're often recognised by the four distinct pale purple gonad rings in the bell. The manubrium (mouth and arms, underside and centre of bell) bears four short, frilled arms. Careful! It has a mild sting.

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Moon jellyfish

Portuguese man-of-war (Physalia physalis)

This animal stings. It isn't a jellyfish, but a floating colony of hydrozoans. The Portuguese-man-of-war has a distinctive oval-shaped, transparent float with a crest. They're blue-purple in colour, with many hanging ‘fishing polyps’ below that may be tens of metres long. Be very careful! They're extremely dangerous to humans due to their powerful sting.

They rarely occur in the UK but should be reported to the local authorities if found in numbers.

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Portuguese man-of-war

Staying safe and respecting our wildlife

Spend enough time at the coast and you are bound to come across jellyfish. While these awesome animals might send some people running for the hills, there's no need to panic. Of the species found in UK waters, most of them are harmless to humans.

One or two species do have a painful sting though, so we'd urge you to avoid touching them – even if they are washed up on the beach.

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.