How to identify types of jellyfish in the UK

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.

(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.

(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.

(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.

(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.

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.

(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.

(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.

(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.