Considered the most venomous creature in the world and one of the deadliest animals in the sea, box jellyfish have a fearsome reputation. If you’re stung by one of the most dangerous species (there are more than 50 types of box jelly), you could be dead within minutes.
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Their transparent bodies make them hard to see in the water, which increases the risk of surfers and other water users getting accidentally stung.
If you do spot them, their cube-shaped bell, which gives them their name, is a tell-tale giveaway that a box jelly is swimming towards you. And, unlike other jellyfish, they actively swim rather than just drifting along in the water.
What are box jellyfish?
Box jellyfish are a type of jellyfish found in shallow, tropical waters and are known for their severe sting, unusual eyes, and strong swimming skills. There are over 50 different species. The Australian box jellyfish (Chironex fleckeri) is the deadliest – with a sting that can kill a human in minutes, it’s considered the most venomous creature in the ocean.
Where do box jellyfish live?
Box jellyfish tend to live in shallow, coastal seas. The most dangerous species are usually found in Australia and the Indo-Pacific. They are usually found in waters shallower than 5m but have been found over 50m deep
What do box jellyfish look like?
As their name suggests, these marine animals have a box-shaped bell. Attached to each corner are long, trailing tentacles covered in thousands of stinging cells. These jellyfish are a transparent blue colour, which makes them almost impossible to spot as they drift towards you through the water.
How big are box jellyfish?
These marine animals come in a variety of sizes. The teeny Irukandji box jellyfish’s 1cm bell is smaller than an ice cube but its tentacles can stretch up to one metre long. On the other end of the scale, the massive Australian box jellyfish is the largest of all the species. Its bell can measure 30cm while its tentacles grow to a maximum of three metres. That’s longer than a horse.
What do box jellyfish eat?
Box jellyfish eat small fish, shrimp, and other crustaceans. While most jellyfish drift along hoping for some tasty morsel to tickle their tentacles, box jellyfish are active hunters. They look for a potential meal with their 24 eyes, then swim over to them to attack.
How do they see?
Most species of jellyfish don’t have eyes, but box jellies have 24. These are arranged in four groups (called rhopalia) of six eyes, with each bunch sitting on a stalk on a corner of their bell. Some of these eyes are simple light detectors while others have a more complicated role.
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Their ‘upper lens eyes’ point upwards to give them a 180-degree view of the world above them and their ‘lower lens eyes’ help them see around them so they don’t bump into obstacles. Scientists are still learning exactly how box jellyfish see the world around them through these 24 eyes.
How do box jellyfish swim?
Unlike many other jellyfish, which float through the ocean at the whim of the currents, box jellyfish can swim using jet propulsion. They have a ring of muscular tissue called the velarium which they expand and contract to open and close the bottom of their bell. This pulsing movement forces water out of their bell and drives them along.
Box jellyfish are relatively strong swimmers. They can chase their prey at speeds of up to four knots (4.6 miles per hour).
How long do box jellyfish live?
Their average lifespan is around one year
Why are box jellyfish so dangerous?
Box jellyfish have a venomous sting to subdue their prey and protect them from predators such as turtles, sharks and fish. Only a handful of box jelly species have a sting that is fatal to humans. The Australian box jellyfish’s venom is so potent that it can kill in a few minutes. Another species – the Irukandji – is also very dangerous and its sting can be fatal.
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It’s believed that more than 40 people (and probably as many as 100 or more) die each year from box jellyfish stings. Knowing exactly how many is challenging as some countries don’t have official channels for reporting a box jelly-related death. For comparison, four unprovoked shark bites proved fatal in 2024.
What happens if you get stung?
The jellyfish’s tentacles contain stinging cells called cnidocytes. Curled up inside a tiny part of these cells are harpoon-like structures called nematocysts waiting for the signal to release their fury. When the box jellyfish’s tentacles come into contact with a living organism, the chemical cue (not the touch itself) sparks a reaction. The stinging cells fire these barbed nematocysts like a dart into the animal that touched them, releasing their toxin. The more of your skin that touches the tentacles, the worse the sting.
This venom is made up of proteins that wreak havoc with your body by creating holes in your cells. It causes pain, scarring, headaches, vomiting, anxiety, sweating, difficulty breathing and increased blood pressure. 14 different species cause a potentially life-threatening condition called Irukandji syndrome, which can cause cardiac arrest. Victims have also reported a sense of impending doom. A sting from an Australian box jellyfish can cause death within five minutes.
What should you do if you get stung?
If you get stung by a jellyfish, remove any visible stingers and rinse the area thoroughly with seawater (freshwater can worsen the sting). To prevent more of the stinging cells being activated, wash the wound with vinegar or hot water (as hot as the patient can stand – but be careful not to scald them. The pain might affect their tolerance to the heat).
Lots of lifeguard stations have vinegar on hand in case of stings so, if you’re on a lifeguarded beach, it’s a good idea to go and ask them for help. Importantly – don’t pee on the area or rub it, this will only make the sting worse.
If you suspect you’ve been stung by a box jelly, call for emergency medical help immediately.
