An underwater photograph of a large, translucent compass jellyfish with brown markings and long tentacles swimming towards the surface, backlit by sun rays.

How to identify types of jellyfish in the UK

Report a jellyfish sighting

    Jellyfish occur throughout UK seas, with large blooms of most species appearing in the spring and lasting through to autumn. Here are some top tips and identifying features to help you decide which species you've spotted.

    Types of UK jellyfish you might discover

    Thousands of citizen scientists, like you, 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're recording, can live in colonies. In colonies, 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've spotted.

    Download our jellyfish ID Guide

     

    Barrel jellyfish

    (Rhizostoma pulmo; Rhizostoma octopus)

    Barrel jellyfish grow up to 1m in diameter. They have a spherical, solid rubbery bell, which can be white, pale pink, blue or yellow and is fringed with purple markings.

    They don't have tentacles, but eight thick, frilled arms hang from the manubrium (found in the underside centre of the bell, with the mouth and arms at its tip).

    Careful! They have a mild sting.

    A barrel jelyfish  |  Image credit: Cathy Lewis

    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.

    Learn more about blue jellies

    Blue jellyfish in Scotland  |  Image credit: Mark Harris

    By-the-wind-sailor

    (Velella velella)

    This isn't a jellyfish, but a floating colony of hydrozoans.

    By-the-wind-sailors grow up to 10cm long and are blue-purple in colour. They have an upright sail with a mass of small tentacles surrounding the mouth on the underside.

    Look out! They occur in vast swarms.

    A close up of a by the wind sailor deflated on a beach

    A washed up by-the-wind sailor  |  Image credit: Graeme MacDonald

    Compass jellyfish

    (Chrysaora hysoscella)

    Compass jellyfish typically grow up to 30cm. The colour varies, but they usually have a pale umbrella-shaped bell with brownish V-shaped markings.

    They have 24 long, thin tentacles and four long, thick, frilled arms hang from the manubrium.

    Careful! This jellyfish stings.

    Learn more about compass jellyfish

    A compass jellyfish  |  Image credit: Mak Kirkland

    Lion's mane jellyfish

    (Cyanea capillata)

    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! This jellyfish stings.

    Discover more about lion's mane jellyfish

    A lion's mane jellyfish  |  Image credit: Peter Bardsley

    Mauve stinger jellyfish

    (Pelagica noctiluca)

    Mauve stingers are relatively small, growing up to just 10cm. They have a deep bell with pink or mauve warts and eight hair-like tentacles. The manubrium has four longer frilled arms with tiny pink spots.

    Be very careful! This jellyfish stings.

    A vibrant pink and purple mauve stinger jellyfish with long, trailing tentacles swims in dark green water

    A mauve stinger  |  Image credit: Peter Bardsley

    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 has four short, frilled arms.

    Careful! It has a mild sting.

    Find out more about moon jellyfish

    A moon jellyfish  |  Image credit: Paul Naylor

    Portuguese man o' war

    (Physalia physalis; Physalia utriculus; Physalia megalista; Physalia minuta)

    This isn't a jellyfish, but a floating colony of hydrozoans. There are four distinct species, of which there are genetically distinct subpopulations; these have been shaped by regional winds and ocean currents.

    The Portuguese man o' war has a distinctive transparent, gas filled float with a crest. They're blue-purple in colour, with many hanging ‘fishing polyps’ below that may be tens of metres long.

    These are 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 large numbers.

    Be very careful! This animal stings.

    An iridescent, translucent purple and pink gas-filled bladder of a Portuguese man o' war floats on the surface of deep blue, undulating ocean water. The bladder, which acts as a sail, has a crimped, fan-like crest along its top edge

    A Portuguese man o' war bobbing in water  |  Image credit: Gonzalo Jara

    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.

    Take our jellyfish quiz

    How well do you know the incredible invertibrates? Put your jellyfish ID skills to the test and learn some fun facts along the way.

    Learn more about jellyfish

    Jellyfish play a huge role in lots of marine food chains and provide habitats for smaller species which shelter under jellyfish bells, protected from predators by their tentacles.

    They are also a great indicator of change in our ocean, so tracking their numbers each year can help us to spot changes in climate, fishing activity or ecosystems.

    An underwater photograph of a large, translucent compass jellyfish with brown markings and long tentacles swimming towards the surface, backlit by sun rays.

    Jellyfish facts

    Less common jellyfish in UK waters

    Our seas are full of jellyfish but there are some species which are less often seen, but just as fascinating. Check out these less widely seen jellies
    An underwater photograph of a large, translucent compass jellyfish with brown markings and long tentacles swimming towards the surface, backlit by sun rays.

    Jellyfish: Helpful hero, not stinging villain

    Jellyfish often get bad press for their stinging sensibilities. But they’re actually remarkable creatures which not only benefit our seas, but the wider environment, too.
    A large, reddish-brown, almost purple, lion's mane jellyfish with long, flowing tentacles and oral arms swimming in blue-green water surrounded by kelp.

    Report a jellyfish sighting

    Your sightings provide vital information about our ocean's inhabitants and contributes to scientific research which finds solutions to protect our seas.

    Report now