Jellyfish swarm British seas in the summer months and often wash up along the shoreline. Learn how to identify, what they eat and what to do if you’re unlucky enough to be stung by one.

By Carys Matthews

Published: Friday, 07 July 2023 at 12:00 am

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Jellyfish are a fascinating – and slightly gruesome invertebrate species. Jellyfish have no brain, no blood, and no heart, and are 95 per cent water. Floating along in the sea for millions of years, fossilised imprints of prehistoric jellyfish unearthed in Utah 2007 from about 540 million years ago tell us their design has barely changed.

Warmer weather conditions caused by climate change is bringing in a wider diversity of jellyfish species to our shores, according to a study by the Marine Conservation Society. Generally, there are six common jellyfish species found in the UK.

Learn more about jellyfish, including how to identify, what they eat and how to treat stings in our expert guide.

Discover more wildlife found on the UK coast with our guides to rockpool species, whales and dolphins and seabirds.

When do jellyfish appear in British waters?

In UK waters jellyfish blooms, or ‘smacks’, peak at the height of our summer, and this is when the adult males release their sperm into the sea. The nearby females in the smack suck the sperm up through their mouth into their stomachs where fertilisation occurs.

The stretch of the Bristol Channel between the Gower Peninsula and the North Devon is the UK’s hottest spot for huge jellyfish blooms.

More related content:

• British seashell guide: how to identify and where to find
• Britain’s best coastal caves, arches and stacks
• Beachcombing guide: things to find along the seashore and best beaches in the UK

What do jellyfish eat?

The opening to a jellyfish’s stomach serves as both mouth and anus. The jellyfish diet is varied and consists of small plants, fish, fish eggs, shrimp and crabs. Some jellyfish species even eat other jellyfish!

Jellyfish tentacles are lined with thousands of stinging cells known as nematocysts. Each cell contains a spring-loaded, toxic harpoon designed to paralyse and secure prey – plankton and small fish.

What eats jellyfish?

Leatherback turtles feed almost exclusively on jellyfish. Each year they perfectly time their epic migrations to UK seas to coincide with peak jellyfish numbers in July and August.

Jellyfish numbers are rising. Studies show that overfishing of plankton-eating fish, such as herring, which predate on the early life stages of jellyfish and compete with the adults, can tip the ecological scales in favour of jellyfish. Collapsing fish stocks have been quickly followed by increasing jellyfish numbers in in the Irish Sea, the South Atlantic and the Mediterranean. Global increases in jellyfish numbers have also been linked to warming sea temperatures through climate change.

What jellyfish species are found in the UK?

Globally, there are thousands of jellyfish species floating in our oceans, however there are just six common jellyfish species that appear in British waters.

Lion’s mane (Cyanea capillata)

Measuring up to 2 metres in diameter, the lion’s mane is one of the UK’s largest jellyfish. Also sometimes known as the giant jellyfish its longer tentacles reach tens of metres in length.

Blue jellyfish (Cyanea lamarckii)

A similar shape and size to the lion’s jellyfish, the blue jellyfish is translucent when young before maturing into a vibrant blue/purple colour. It can measure up to 30cm across and tends to appear in UK waters in late summer and autumn.

Compass jellyfish (Chrysaora hysoscella)

Compass jellyfish often wash ashore in the summer months so be careful when walking barefoot along the shoreline as this common species can give a nasty sting. This translucent yellowish-white jellyfish has brown markings which resembles a compass.

Barrel jellyfish (Rhizostoma pulmo)

The enormous barrel jellyfish is the largest in the UK. Sometimes nicknamed the dustbin-lid jellyfish thanks to their size, the barrel jellyfish swarms the British coastline in May and June when warmer waters entice them in. Their diet tends to consist of plankton.

Moon jellyfish (Aurelia Aurita)

Resembling a moon in appearance with four distinctive circles on its white milky dome, the moon jellyfish is roughly the size of a dinner plate and lives for around six months. Found worldwide it reaches the UK’s shorelines in the winter months.

Mauve stinger jellyfish (Pelagia noctiluca)

A toxic jellyfish with a powerful sting, the mauve stinger jellyfish is one you want to avoid while swimming. One of the smaller species of jellyfish, it is impressive looking with a bright purple/yellow dome which is covered in orange warts. You’re less likely to spot this jelly close to the UK shore as it tends to float further out at sea or in warmer waters.

What does a jellyfish sting look like?

Jellyfish stings will leave a rash and can be quite painful with a burning sensation. The sting tends to be line of red, raised welts or blisters which will leave an itchy rash.

How to treat a jellyfish sting?

Jellyfish found in the UK don’t tend to be dangerous, unlike the deadly Box jellyfish which is found in Australian waters and injects a poison as it sting. That said, a jellyfish  sting can be painful, so it is worth taken some caution when you swim in the sea and avoid times and areas where jellyfish are swarming.

Here’s what to do if you are stung by a jellysfish:

1. Firstly, do not panic. A nasty sting can be a shock to the system but try to stay calm – especially, if you’re out of your depth.

2. If you are in the water get out, and thoroughly rinse the affected area with sea water immediately. Do not use urine. Wash the area with clean, warm water to reduce the chance of infection.

4. So long as the stinging tentacles are on you they will continue to inject you with the toxin, and worsen the effect. If you can see them, remove them with a stick or tweezers.

5. Apply an ice pack if available.

6. For the medical advice see: nhs.uk/Conditions/Stings-marine-creatures/Pages/Treatment.aspx or seek a medical specialist straight away if the pain increases and you experience shortness of breath or chest pains.

 

 

 

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