Jellyfish are common sea creatures and many of them sting. Most jellyfish stings are not harmful. Jellyfish species in the UK are generally not dangerous, although stings from the Portuguese man o' war, Lion's mane jellyfish and compass jellyfish can be painful and, if stung many times, harmful. It is also possible to have an allergic reaction to any jellyfish sting.
What are jellyfish?
Jellyfish are soft-bodied free-swimming animals with a jelly-like umbrella-shaped bell and trailing tentacles. The tentacles are covered in tiny sacs (nematocysts) which contain venom. A few live in fresh water, some in shallow coastal waters, and some in the deep oceans. They are believed to be the oldest multi-organ animal species, dating back over 500 million years.
Do all jellyfish sting?
Not all jellyfish sting. Of those that do sting, very few are dangerous to human beings. There are more than two thousand known species of jellyfish, but it is estimated that three hundred thousand are yet to be discovered.
Most jellyfish stings result from swimming into jellyfish, wading into them in shallow water, walking on them (or parts of them) on the beach, or attempting to handle them. Swimmers are at the greatest risk; most ocean distance and endurance swimmers regularly experience repeated jellyfish stings.
Are jellyfish poisonous?
This depends on what you mean by the term poisonous. Most UK jellyfish can sting, although none is dangerous to life. Some have very painful stings - but the term 'poisonous' is usually reserved for toxins which can cause us real and lasting harm.
Box jellyfish are a large group of jellyfish species. The most dangerous varieties are found in the Indo-Pacific, and these have venom which is definitely poisonous. They are very different to the jellyfish found in UK waters, as they are able to see, and to propel themselves, and they can sting when in close proximity even without being touched. Some of these jellyfish have extremely venomous stings which can be a danger to life.
What kinds of jellyfish are found in the UK?
The UK has six species of true jellyfish and two species of jellyfish-like animals, the Portuguese man o' war and the by-the-wind sailor.
Common jellyfish are found in all UK waters, particularly in estuaries and bays, and are mainly seen from mid spring through to mid summer. They are transparent, with pale pink or orange tentacles, and up to 30-40 cm in diameter. Their sting is extremely mild.
Compass jellyfish (or dial jellyfish)
These are clear with orange-brown stripes radiating out from the centre, like the points of a compass. Up to 30 cm across, they have 24 long thin tentacles and four frilled dangling 'arms'. They are found mainly in the southern half of the UK in summer. Although they are not dangerous, their sting can be quite painful, like a bad nettle sting.
These are bright blue/purple with a frilly edge and long, fine tentacles. They are common in the South West and Wales in spring and summer, and have a mild sting.
Lion's mane jellyfish
These much larger jellyfish prefer colder waters, and are found mainly from North Wales to the north of Scotland (and further north). The frilled bell is reddish-brown and can be up to two metres in diameter. The multiple very fine tentacles trail for 10-20 metres and have a painful sting. Bits of tentacles can break off the animal and drift in the tide, where they may still sting. The sting is sometimes describes as feeling like an electric shock.
These pale creamy-pinkish barrel jellyfish have a solid, rubbery bell up to one metre in diameter, with purplish markings around the edge. They are found mainly on the Western side of the British Isles in summer and autumn. They do not have tentacles but eight thick, frilled cauliflower-ish arms, and have only a very mild sting.
Mauve stinger jellyfish
These small (10 cm) Mediterranean jellyfish are covered in pink or mauve warts, and have four long frilled arms. They are rare in the UK, but common in the Mediterranean in summer. They have a painful sting, sometimes described as like an electric shock or sometimes like a nettle sting.
Portuguese man o' war
The Portuguese man o' war, Physalia physalis, is not a jellyfish but a group of many different individual animals called zooids, attached and co-dependent, so that they behave as one animal. Found in the Atlantic, Indian and Pacific Oceans, these creatures travel in large groups, sometimes of hundreds of thousands, so where one man o' war is beached others often follow. In the last few years there have been several reports of groups of the creatures drifting off British shores and blowing up on to beaches. Like most jellyfish they float on the sea's surface and drift with the wind and currents - they can't propel themselves.
The man o' war is translucent with a purplish tinge, and the 'body' can be up to 30cm long, but the tentacles are typically ten metres long and can reach thirty metres or more. The visible part is a gas-filled bladder which is visible above the water and acts as a sail. The tentacles have venom-filled nematocysts which sting, paralyse and kill small fish on contact. These cause a very painful sting in humans, sometimes described as feeling like a sudden burn with a hot whip. Tentacles of parts of tentacles frequently break off and may drift for days, still venomous, with the potential to sting swimmers. Once beached and apparently inert, the man o' war still stings, since the cysts on the 'tentacles' will be alive for some time.
Deaths from the man o' war are possible but vanishingly rare - two have been reported in the last fifty years, neither in UK waters. In both cases the victim appears to have had an allergic reaction (anaphylaxis) to the sting.
The by-the-wind sailor, velella velella, is a group of organisms like the Portuguese man o' war. It is blue with a distinctive triangular sail on top of a circular base under which hang small tentacles. It can occur in vast swarms and huge numbers are occasionally stranded along the South West and Welsh coastlines. Unlike the man o' war, it is harmless.
How can I avoid jellyfish stings?
Avoid touching jellyfish. It is generally better not to touch a jellyfish at all, even if you think it is a harmless variety and even if it is beached and appears dead, as it may still sting.
If you see jellyfish in the water, swim away and be aware that jellyfish are generally found in groups.
A wetsuit will protect you against most jellyfish stings, although exposed parts of your skin like hands, face and feet will still be stingable.
What does a jellyfish sting feel like?
Symptoms include a localised stinging pain, itching, rash, and raised stripes (welts). For the vast majority of people stung in UK waters this will be the experience of a jellyfish sting. The initial intensity varies from something like a nettle sting to something like a sudden burn to the skin, although man-o'-war stings can then become more sore before the pain settles down.
A few people go on to feel sick, be sick (vomit), develop diarrhoea, have tummy (abdominal) pain, experience numbness/tingling and have muscle spasms. In the UK this is more likely if you have multiple stings from a lion's mane jellyfish, compass jellyfish, mauve stinger or the Portuguese man o' war.
Allergic reactions to jellyfish stings are unusual but may cause swelling, chest tightness, difficulty breathing and anaphylaxis. Anaphylaxis is a medical emergency - see the separate leaflet called Anaphylaxis.
What is a Portuguese man o' war's sting like?
The sting of the man o' war is very painful, and is often described as feeling like a sudden burn on the skin. There are around 10,000 human stings caused by the man o' war in Australia each summer, mostly on the east coast.
Stings are normally multiple, causing skin wheals longer than 20 cm. Some severe skin injuries turn into scabs lasting for about two weeks before resolving and leaving red scars. Stings cause pain lasting 2-3 hours. Wheals are usually extremely sore, and leave red open welts on the skin that last 2-3 days.
Less commonly the venom can trigger a reaction rather like an allergy, including swelling of the airway, heart rhythm disturbance and breathing difficulty. Other symptoms can include high temperature (fever) and shock. Fatal stings are extremely rare, and usually result from allergic reaction to multiple stings.
Should I pee on a jellyfish sting?
Do not urinate on a jellyfish sting. The contents of your bladder are variable - they may be acid or alkaline. If they happen to be alkaline you will cause more toxin to be released and make the sting and the pain worse.
How do I treat a jellyfish sting?
It is useful to know what to do if you get stung by a jellyfish in the UK, although if you swim in tropical seas, particularly the Indo-Pacific, it is vital to have this information and to be prepared. As soon as possible, remove any remaining tentacles, without touching them with bare skin. Use a stick. If you use items of clothing or towels, don't wear or use them afterwards.
- Rinse thoroughly in seawater to wash off all the cysts you can. Do not use fresh water, as this may make any jellyfish parts remaining leak further venom.
- After rinsing thoroughly, soak or rinse the area in vinegar (acetic acid) for about 30 seconds. This will prevent the cysts from releasing their toxins by disabling them.
- If you do not have vinegar available, continue to rinse in sea water, or commercial jellyfish pain relief gel.
- Do not rub the area yet as there may be cysts clinging to your fine body hairs and you will break them and potentially will sting your hands.
- Do not yet apply ice or hot water.
- Apply shaving cream to the area. Shave the area with a razor or credit card to remove any adherent cysts. Then reapply vinegar or alcohol. The shaving cream prevents cysts that have not been activated from releasing their toxin during shaving.
- Eye stings should be rinsed with a commercial saline solution like Artificial Tears®; dab the skin around the eyes, using a towel that has been soaked in vinegar. Do not place vinegar directly in the eyes. If you are on the beach, use sea water.
- Mouth stings should be treated with ¼-strength vinegar. Mix 1 to 3 parts vinegar to water. Gargle and spit out the solution.
- If the person who has been stung starts to feel general symptoms which are not local to the sting, such as feeling sick, being sick (vomiting) or tummy (abdominal) pain, seek medical advice urgently.
How do I treat a sting from a Portuguese man o' war?
Treatment is on the same principles as other stings. It aims first at removing remaining cysts from the skin, where they may be trapped by body hairs. Start by pouring lots of salt water on to the skin. Avoid fresh water or alcohol, both of which can make cysts burst, releasing more venom.
Rinse thoroughly with vinegar for about 15-30 seconds to deactivate the remaining cysts and help neutralise the venom and the pain.
Rinse for a further 15 minutes. If you can, apply shaving cream to the wound for 30 seconds, then shave the area with a razor (rinsing between each stroke). This is the best way to remove remaining cysts trapped in or clinging to the skin's hairs. If you don't have a razor, use a credit card to scrape the skin, then rinse again and reapply vinegar for another 15-30 seconds.
Only apply heat or cold AFTER this. Hot packs then speed the breakdown of the man-o'-war toxins already in the skin.
In a trial on 96 swimmers accidentally stung by Physalia, hot water immersion at 45°C was better than ice, although ice can be helpful.
Topical vinegar is not universally accepted as treatment for Portuguese man-o'-war stings in Australia, as it can worsen stings from a similar multi-tentacled Physalia species living in Australian waters. However, the multi-tentacled species has not been reported outside Australia, and vinegar remains the recommended way to manage jellyfish stings elsewhere.
Where are the most dangerous jellyfish found?
The most dangerous and indeed deadly species of jellyfish, where humans are concerned, are some of the box jellies found in the Indo-Pacific ocean and around the coasts of Australia, particularly to the North, and the Philippines.
Box jellyfish are pale blue and transparent with a cube-like bell. Each has up to 60 tentacles, bearing thousands of stinging cells. Firing is triggered not by touch but by sensing chemicals on the outer surface of prey.
Box jellies can move purposefully rather than just drift, travelling at up to four knots, and hunting for food. They have clusters of eyes on the four sides of their bell, with pairs of eyes on each side possessing a lens, retina, iris and cornea, although as they have no central nervous system, it is unclear how they process what they see.
There are multiple box jellyfish varieties. Not all are dangerous, but the best known (and most dangerous) are the huge Chironex whose venom affects the heart, and the tiny Irukandji, whose venom can cause Irukandji syndrome, affecting the heart, nervous system, and skin.
What is a box jellyfish sting like?
Box jellyfish stings are very painful. The box jellyfish fires its venom and cysts into the skin, greatly increasing their harmful potential. One of the reasons for the particular hazardous nature of box jelly stings is that those stung can go into shock, rendering them unable to swim, so that they drown before reaching shore.
Chironex fleckeri is a very large box jellyfish, with a venom that can cause heart rhythm disturbance and cardiac arrest. Stinging from contact with four feet of tentacle can kill an adult man, and two feet can kill a child. Smaller stings are not usually fatal. About 40 people a year die in the Philippines from box jellyfish stings. In Australia, where beaches are netted and public awareness is high, fewer than 100 people are thought to have died from box jellyfish stings since 1883, although some unexplained swimming deaths may relate to the creature.
Chiropsalmus quadrigatus is a slightly smaller, very common and equally dangerous species in the Indo-Pacific Ocean, also found less frequently in Caribbean seas. It is believed to have caused large numbers of fatalities in pacific waters, but not in Australia.
Irukandji jellyfish are small - measuring only 1-3 cm across (depending on the type), but the barely visible tentacles may be a metre long. They inject venom and cysts into the skin, causing Irukandji syndrome. This involves severe muscle pain including severe low back pain, rapid heart rate, massively raised blood pressure, nausea, headaches, sweating, muscle numbness, chest tightness, restlessness, sickness (vomiting) and a terrible sense of doom. In severe cases they have been known to cause brain haemorrhage. There is a delay of around 30 minutes before symptoms begin.
About 100 people are hospitalised each year in Australia due to Irukandji syndrome. A single sting is not usually fatal, although multiple stings from a group of jellies are extremely dangerous. Fatalities are very rare, but hospitalisation with induced coma can be needed for up to two weeks to manage symptoms. Survivors may experience considerable pain for weeks, and are usually left with permanent scarring where the tentacles made contact.
Fear of box jellyfish closes almost all of the beaches of Northern Australia. The few beaches that remain open have jellyfish netting, and lifeguards wear neck-to-ankle Lycra® suits. Signs warn swimmers not to rub a sting, but instead to douse it in vinegar.
How do I treat a box jellyfish sting?
Most beaches where these jellyfish are found have warning signs for marine stingers, so treat any sudden sting as if it's a box jelly in these circumstances.
- Immediately flood the area with household vinegar to keep un-discharged nematocysts from firing. This does not relieve pain but prevents additional stings.
- Never rub the area with sand.
- Pluck off any vinegar-soaked tentacles with a stick or other tool.
- If the victim has shortness of breath, weakness, muscle cramps, the sensation of having a thumping heart (palpitations) or any other generalised symptoms, call for emergency medical help.
- For pain relief, apply ice packs. If pain becomes unbearable, seek medical advice. No studies support applying heat to these stings.
- Meat tenderiser, baking soda, papaya, commercial sprays, alcohol, and urine are not effective. They often have harmful effects, causing more damage.
- If a red streak develops between the sting site and neighbouring lymph glands, or if either area becomes red, warm and tender, seek medical help immediately.
- Do not delay in calling for emergency medical help if the victim shows signs of a significant reaction.
- Medical treatment is with antihistamines, strong painkillers and medicines to lower blood pressure.
- There is some evidence that strong local anaesthetic solutions help the pain of some box jellyfish stings.
Are there any box jellyfish in the UK?
Box jellyfish live in warm waters, not the colder waters of the UK. This information on box jellyfish is included because people are interested in them and often want information. If you visit the Pacific or Australia, take note of local advice regarding swimming and do not swim on beaches which are closed due to hazardous marine life.
Very painful jellyfish stings are rare in UK waters, and there are no records of anyone ever being killed by jellyfish in the UK. Fatal jellyfish stings are incredibly unlikely. Be sensible; avoid jellyfish if you see them and consider wearing a wetsuit if you like to spend time in the sea. If you see large numbers of jellyfish drifting together (sometimes called a bloom) then get out of the water, but don't let the idea of jellyfish prevent you from enjoying exercise and the ocean.
Increasing sea temperatures are likely to affect where jellyfish are found. There are reports from Australia of box jellyfish being found further south, round the Australian coast, than in the past, but these particular jellyfish remain in the wrong part of the world's oceans to represent a possible threat in the UK. Whilst there are varieties of box jellyfish in warmer Atlantic waters much nearer to the equator than us, the main risk to swimmers in the UK is not from sea life but from currents and tides.
Always take note of local warnings regarding swimming conditions before venturing into the sea.
Further reading and references
Cegolon L, Heymann WC, Lange JH, et al; Jellyfish stings and their management: a review. Mar Drugs. 2013 Feb 2211(2):523-50. doi: 10.3390/md11020523.
Prestwich H, Jenner R; Best evidence topic report. Treatment of jellyfish stings in UK coastal waters: vinegar or sodium bicarbonate? Emerg Med J. 2007 Sep24(9):664. doi: 10.1136/emj.2007.052290.