Spineless killers

Sheree Marris | VOLUME 31, ISSUE 1

Triggering fear among swimmers from coast to coast, box jellyfish carry an arsenal of frighteningly powerful venom.

It’s a box with four corners and 24 eyes. It’s not the type you would want to pack a present in, though, because the eyes on this box are those of the deadly box jellyfish.

This group of animals has a fearsome reputation, and with good reason: they can kill an adult in less than three minutes. And you won’t see them coming until it’s too late because they’re transparent.

But rather than fostering fear about them, we should be fascinated by these creatures. Box jellyfish are swimming fossils. They’ve had over 500 million years to refine their survival tactics and, along with sponges, are considered the oldest multi-organ animals on earth.

Box jellyfish aren’t your average jellyfish that just float around – they’re much more sophisticated. Anything other jellies can do, they can do better and with greater impact. They swim faster than an Olympic swimmer, create their own weapons, consist of 96 per cent water and are the deadliest animals on earth.

The main physical feature of box jellyfish is the trademark box-like body (or bell) which has long tentacles hanging from each corner. These high-tech, multitasking tentacles can change length, lure prey, digest food and excrete waste. What they do best, though, is incapacitate their victims.

Its tentacles are laced with billions of venomous harpoons called nematocysts that are coiled up inside the arms. It’s estimated there are around 500,000 harpoons per square centimetre inside each tentacle. When triggered, the harpoons shoot out and penetrate the prey’s skin.

The harpoons are miniscule, measuring around 10 microns (thinner than hair), which begs the question: how can something so small be strong enough to penetrate skin?

It’s all due to speed. The stinging process takes just 0.0000009 seconds from trigger to game over. That’s 900 nanoseconds (billionths of a second). It’s the quickest biological mechanism known.

Once the victim’s skin has been pierced, the most potent venom known to mankind is pumped into its skin, paralysing the prey. It’s then passed up to the mouth, which is on the inside of the ‘box’ and transported into the stomach, where it’s partially digested before being passed back down the inner section of the tentacles. These function as intestines, processing the food and absorbing all the fishy goodness. The waste is then simply passed out the end of the tentacles.

Unbeknown to many, box jellyfish are active visual hunters – they have clusters of eyes on the bottom of their box-like body, which includes both light-sensitive and more complex eyes with a lens, retina and pupil. Like our eyes, the pupil reacts to light intensity, giving them depth of field so they can see things in three dimensions. They can also tell how close objects or other animals are, helping them hunt for prey. Even more amazing is that its eyes all face inwards, so the animal can look through itself as it is transparent … they have 360-degree vision!


Two of the most notorious box jellyfish are the big box and the Irukandji (pronounced ira-can-gee) jellyfish.

As the name suggests, the big box jellyfish is the biggest blob in this fascinating family. It measures around 38cm (that’s just the boxy part) with 15 flat, fettuccini-like tentacles delicately hanging from each corner of its box head. These tentacles can measure 3m in length, which equates to a whopping 180m of impressive firepower that would turn any fish into jelly.

More than 3m of tentacle contact with this box will also see you go home in a box – its venom can kill humans quicker than any other. What’s really interesting is that they only become dangerous to humans as they age: when big box jellyfish grow to the size of a fist, their diet changes from invertebrates, such as shrimps, to more complex vertebrate animals, such as fish. Given that humans are also vertebrates, the venom is dangerous.

Unlike its bigger counterpart, the Irukandji jellyfish is slow and small, measuring only 2.5cm, and has only a single tentacle hanging from each corner. But dismiss this jelly at your peril, as any contact with its tentacles will see you in hospital.

Irukandji tentacles can be 40-times the length of its body. They’re covered in bright little balls and by quickly extending and contracting, they act like a lure to attract fish. Any fish foolish enough to take the bait gets a face full of pain, as there are clusters of tiny, venom-filled harpoons that discharge on contact.

Box jellyfish live in tropical and sub-tropical oceans worldwide, but mostly the Indo-Pacific region. In Australia, they’re found in coastal waters and around mangroves, estuaries and sandy shores off Northern Australia, from Rockhampton on the east coast to Exmouth on the west coast.

World-leading venomous and dangerous animals expert, Associate Professor Jamie Seymour of James Cook University, Qld, has been studying these animals for around 20 years. His current research involves exploring the novel toxins from box jellyfish venom – that is, how we can use the venom. Research to date has revealed that non-toxic components of the venom can have impacts on weight loss, assist with irritable bowel syndrome, and treat arthritis. That’s impressive for an animal that is little more than water with a bit of thickener added.


Jamie Seymour has had his fair share of stings while researching box jellyfish and is more than happy to share his tips on how to protect yourself from these venomous creatures when you’re in the water.

• Be aware of the seasons for box jellyfish. In Australia, this is usually from November to May

• Watch out for signage near the water and alerts issued by health authorities, local councils and in the news

• If you’re going to be in the water, protect yourself by wearing a Lycra suit. Even the thinnest material can provide a barrier between a barrage of venom-filled harpoons and your skin.


Current first aid recommendations suggest the use of vinegar. If the patient isn’t breathing, call 000 and commence ‘DR ABC’:

Danger – is there any danger to the person doing the first aid

Response – is the patient responding

Airways – does the patient have an open airway

Breathing – are they breathing

Circulation – is their heart beating? If not, commence CPR.

Seymour has dispelled the myth about urinating on stings – his advice: don’t do it. Research suggests that this actually makes the sting worse, as chemical components in urine cause more of the stinging cells to discharge, which means more venom into the victim’s system.

Creature Features