The most venomous creature in the ocean is the Australian box jellyfish. Its painful sting causes headaches, vomiting and difficulty breathing, and can be fatal to humans in minutes.
There are more than 50 species of box jellyfish, which are sometimes also known as ‘sea wasps’. Not all of these have a deadly sting and, thankfully, none are currently found in UK waters. They prefer warm tropical climates like Australia and the Indo Pacific. But if the ocean's temperature keeps rising, they may be forced to migrate.
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What are box jellyfish?
Typically found in tropical waters around the world, box jellyfish are cnidarian invertebrates known for their dangerous sting and box-shaped body. There are over 50 different species of box jelly and not all of them have a fatal sting. However, the Australian box jellyfish (Chironex fleckeri) can kill a human in a matter of minutes. It is thought to be the most venomous animal in the ocean.
What do box jellyfish look like?
Box jellyfish can be hard to see in the water because they are translucent, which helps them blend into the background. They have a box-shaped bell – hence the name – with stinging tentacles attached to each corner.
How big are jellyfish?
The different species of box jellyfish come in a range of sizes, from the Irukandji, which has a 1cm bell and tentacles of up to one metre long, to the Australian box jellyfish with its 30cm bell and three-metre tentacles. This makes it longer than a horse. They can weigh up to 2kg.
Do box jellyfish have eyes?
Yes. Most species of jellyfish don’t have eyes, but box jellies do. In fact, they have 24. These are clustered together with six eyes in a group at the corner of each bell. The little eye-bundles (known as rhopalia) sit at the end of special stalks.
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Researchers are still studying exactly how a box jellyfish’s eyes work. They know that some are quite basic and just sense light while others can form images. These jellies have a 180-degree view above them thanks to their ‘upper lens eyes’ while their ‘lower lens eyes’ help them see obstacles in the water.
Special features of a box jellyfish
- Square bell
- Mouth
- 24 eyes
- No brain, heart or blood
- Velarium
- Cnidocysts (stinging cells)
- Nematocysts (that inject venom)
- Four pedalium (stalks)
- Long tentacles
What do box jellyfish eat?
Unlike other types of jellyfish, which can’t control where they swim, box jellies are active hunters. They eat small fish, shrimp, copepods, and other crustaceans.
How do they swim?
Box jellyfish are unusual because they can swim – at speeds of up to four knots (4.6 miles per hour) – rather than just floating along aimlessly in the current. They are powered by jet propulsion: expanding a contracting a ring of muscular tissue (the velarium) around the bottom of their bell to force water out of their bodies when they want to move.
Why are box jellyfish so dangerous?
The most dangerous species is the Australian box jellyfish. It can kill a person in under five minutes. The Irukandji jellyfish is also very dangerous. Between 40 and around 100 people are thought to die each year from box jellyfish sting. This is much higher than the number of fatalities from unprovoked shark bites, which was just four in 2024.
How do box jellyfish stings hurt you?
Box jellyfish use their sting to hunt and as self-defence. Their tentacles contain thousands of stinging cells called cnidocytes. Inside a small part of these cells are structures called nematocysts. These are like tiny, barbed harpoons, waiting to strike. When a jellyfish’s tentacle touches a living thing, the chemicals cause the nematocysts to shoot out and fire their venom.
This toxin is made up of proteins that attack your cells, creating holes and causing them to deteriorate. Symptoms of a sting include pain, scarring, headaches, vomiting, anxiety, sweating, difficulty breathing and increased blood pressure. In some instances, victims feel a sense of impending doom and some are in so much pain they want to die. In the worst-case scenario, the venom can cause cardiac arrest and death.
The more of your skin that touches the tentacles, the more severe the sting. Rubbing the area can make it worse because the action sets off any nematocysts that hadn’t yet fired, causing them to add their venom to the already painful cocktail.
Should you pee on a jellyfish sting?
The myth that you should pee on a jellyfish sting is not only gross, but also unhelpful. Peeing on a sting is worse than useless: it can exacerbate the situation by encouraging the stingers to release even more venom. The same goes for rubbing the area – this sets off even more stingers.
If you’ve been stung by a jellyfish, try to remove any tentacles or stingers you can see attached to your skin and rinse the area with seawater (not freshwater – this can also make things worse). Applying vinegar or hot water (as hot as the patient can stand without scalding them) can also help to deactivate the stinging cells so they don’t fire any more of their venomous harpoons.
Are box jellyfish found in the UK?
No. Box jellyfish aren’t typically found in UK waters. They thrive in shallow, tropical seas with warm waters. As sea surface temperatures continue to warm around the world, the distribution of box jellyfish and other marine animals may change as they follow suitable conditions and prey that allow them to survive. Will they ever come to the UK if ocean temperatures keep rising? It’s not impossible.
Could they be mistaken for other jellyfish in UK waters?
Other species of jellyfish are found in the UK, including the barrel (Rhizostoma pulmo), compass (Chrysaora hysoscella), moon (Aurelia aurita), blue (Cyanea lamarckii), and lion’s mane jellyfish (Cyanea capillata).
The easiest way to tell the difference between these species and the box jellyfish is by looking at their bell. Most jellyfish have an umbrella-shaped body but box jellies are noticeably cube-shaped rather than domed.
Although none of the jellies that typically inhabit UK waters are lethal, some of them can sting. If you see a jellyfish in the sea, it’s best to steer clear – just in case.
How long do box jellyfish live for?
The average lifespan of a box jellyfish is around one year
Main image: an Irukandji box jellyfish/Getty
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