A Wild Swimmer's Guide to Jellyfish and other Sea Stingers

A Wild Swimmer's Guide to Jellyfish and other Sea Stingers

For those of you who have been sea swimming of late, you may have noticed some jellyfish about in our coastal waters.  Every year we have to brush up on our knowledge and jellyfish ID to be able to identify the different species of jellyfish, so we thought we’d put together this little run down of the different jellyfish species we get in British waters. 

Did you know?

  • A group of jellyfish is called a ‘smack’ or a ‘bloom’ or a 'swarm'.
  • A 'bloom' refers a dense cloud of jellies caused by an actual spike in reproduction whereas a 'swarm' refers to jellyfish that collect in one area due to a result of strong winds or currents.
  • Jellyfish have no eyes but can sense light, they don’t have a sense of smell, a brain or a skeleton.
  • They have been around for millions of years, even before the dinosaurs.
  • Jellyfish can sting even when they are dead.

Moon jellyfish (Aurelia aurita)

Moon Jellyfish washed up on British Beach Aurelia Aurita

Moon jellyfish are the most common jellyfish in British waters and can be identified by the four round gonads through the translucent white bell.  They swim by pulsations of the bell-shaped upper part of the animal.

They have short delicate tentacles that hang down, their tentacles are covered in specialised stinging cells, called cnidocytes which they use to hunt small small invertebrates and to capture other food particles like zooplankton.  It is these stinging cells which can sting us humans, but the sting is often mild and more often than not you won’t feel it. 

Animals which eat moon jellies include the magnificent sunfish (Mola mola) which can be found in British waters, however the moon jellies are of little nutritional value so they have to eat hundreds and hundreds of them!

Moon jellies are not very strong swimmers which is why they are often found in large numbers on beaches after storms or strong tides.

A swarm of Moon jellyfish washed up on a beach in Cornwall

Blue jellyfish (Cyanea lamarcki)

Blue jellyfish in British Seas in Cornwall underwater

The blue jellyfish has a dome shaped bell with its tentacles trailing underneath.  They can sting, so if you come across one in the sea then its best avoided. Most reports say it is innocuous and we've heard from swimmers who have been stung that it is comparable to a stinging nettle sting.

Blue jellyfish can be seen in our waters from May to October and they are often attracted inshore plankton blooms on which they feed on.

You can tell younger and older individuals apart by their colour, the darker they are the more mature and lighter coloured ones are younger.  Although, beware, the paler, younger ones can often be confused with a lion’s mane jellyfish . 


Compass jellyfish (Chrysaora hysoscella

Compass jellyfish

Photo by Ryoji Hayasaka on Unsplash

These beautiful jellyfish are easily recognisable by compass like brown markings on the top of their yellow-white bell.  They are found in our waters from May to October. 

Compass jellyfish feed on small fish, crabs and other jellies and amazingly even though this jelly predates on small fish,  fish can often be seen seeking shelter in amongst their tentacles!

If you do see one whilst swimming in the open water, make sure you keep your distance as they give a nasty sting.


Barrell jellyfish (Rhizostoma pulmo 

Barrell jellyfish washed up on a beach in Cornwall

Barrell jellyfish are the largest species we get in the UK and can grow up to the size of a dustbin lid which is where they get their other common name from, dustbin-lid jellyfish!

They can be found in our waters from May to October.  These jellyfish feed on plankton, and will come into shallow waters following the plankton which is often when they end up washing ashore.

The good news is though, despite their size their sting isn’t normally harmful to humans but we’d still advise to be careful around them.


Lions mane jellyfish (Cyanea capillata

Lions mane jellyfish
Photo by Willow and Kevin

The Lions mane jellyfish has long flowing tentacles around its bell and from this  you can see where they get their name from. These long tentacles are covered in stinging cells which they use to catch fish and other smaller jellies.

They can be seen between May to October and can be found off all UK coasts in the summertime.

The Lion’s Mane jellyfish is the longest recorded jellyfish in the world. It can grow up over 30m in length (although this is not common).

If you spot one of these, keep well away, they have a very nasty sting. If you do get stung then scrape the area where you are stung with a card, clean stick or tweezers to remove the tentacles. Also rinse the area with hot water and consult a doctor if it is a severe sting.


Crystal Jellyfish (Aequorea victoria)

Crystal jellyfish
Photo by Mick Haupt on Unsplash

Crystal jellyfish are uncommon in British waters, however we have heard of quite a few reports of them on the South Coast of Cornwall this year so far. 

The crystal jellyfish is a beautiful transparent jellyfish which are really a delight to witness in the water. They can glow in the dark and glow bright green when agitated. Amazingly they can eat jellyfish bigger than themselves but often opt to eat small copepods. 

They can sting humans but it doesn't often cause anything more than some redness and itchiness. 

    Portuguese Man-o-war (Physalia Physalis)

    Portugese man o war

    You would be mistaken to think that a Portuguese Man-o-war was a jellyfish but it is in fact what is called a siphonophore, a hydrozoan made up of a colony of several small specialized organisms called zooids that work together as one.  Each zooid has its own job to do like feeding or breeding. They are so closely intertwined that they cannot survive on their own. 

    They cannot move on their own but use the wind and ocean currents to move. They can be seen from September to December and can be found washed up on our shores after westerly winds, like the one shown in the photo above. 

    Steer clear of them, their tentacles have a really really nasty sting and the tentacles can stretch as much as 20m in length.  So if you spot the telltale 'ship-like' gas filled bladder at the surface of the water, keep a wide berth as their tentacles will be lurking!

    Weaver fish (Echiichthys vipera)

    Weaver fish have an almighty sting, with being one of the only venomous fish to be found in British waters.  They have an upturned, grumpy mouth, with a deep yellowish body tapering down towards the tail with their eyes on the top of their head. 

    It is their dorsal fin, the fin which sticks upwards that contains the venom, along with the spines on their gill covers.   They are difficult to see as they bury theirselves under the sand and their body colour looks similar to the sand, which is why it's so hard to see if you're going to stand on one!  When they are disturbed they will shoot up their black dorsal fin to defend themselves, and if you have stood on one, this venom will be injected into your foot. 

    They can be found from June - October, usually in shallow waters around low tide. 

    What to do if you are stung by a weaver fish:

    • If you are on a beach with Lifeguards, then seek out their help
    • Place the body part in as hot water as you can bear, as quickly as possible for up to 90 minutes

    To avoid being stung, we would advise wearing beach shoes when paddling in sandy waters! Another tips of ours is to stamp your feet when walking out, so that the fish know you are approaching, they don't want to be stood on so will scamper if they know you are approaching.

    On one of our favourite beaches they are often found on the edge of the river and when it’s low tide.  Our Mum has been stung twice on the bottom of her foot in the past and describes it as “a searing pain which quickly got worse and worse”  so much so it was hard to walk, so she had to hobble up the beach to the lifeguard hut where they plunged her foot into boiling hot water. 


    What to do if you get stung by a jellyfish or other sea stingers

    We would advise familiarising yourself with the information on the following NHS page, which includes up to date advice on what to do if you get stung by a jellyfish.


    If you spot a jellyfish then report it to the Wildlife Trust 

    Your local Wildlife Trust wants to know when you see a jellyfish.  When reporting please provide date, location, number (and ideally a picture) information for the accurate creation of sighting records.











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