When people worry about diving into the ocean, sharks are usually the first animal that spring to mind, even though there is an incredibly low risk of a negative shark encounter (there are just six unprovoked fatalities each year).
The most dangerous animal is actually much smaller: the Australian box jellyfish (Chironex fleckeri). Of the 50 box jellyfish species, at least eight can kill humans and these are typically found in Australia and the Indo-Pacific.
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The Australian box jellyfish can kill an adult human in just five minutes – and it’s not pleasant. The toxins from a sting cause severe pain, vomiting, breathlessness and even a feeling of pure dread.
These cube-shaped jellies don’t just float through the water at the mercy of the currents like other jellyfish. They can actually swim through the water using jet propulsion, reaching speeds of nearly four knots (4.6 miles per hour). And you might not even see them coming: their blue bodies help them blend in perfectly with the water around them.
They have 24 eyes but we don’t fully understand how they see the world around them.
Other highly venomous box jellies include the Irukandji jellyfish (Malo spp.) and Barnes’ jellyfish (Carukia barnesi). Compared to the large Australian box jellyfish, which can grow three-metre tentacles and a 30cm bell, these are much smaller. Their bell grows no larger than 2.5 centimetres, although their trailing tentacles stretch behind them for a metre.
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They’re so hard to see in the water that, for many years, experts didn’t know what was causing thefatalities. In 1961, scientist Jack Barnes finally put the issue to bed by stinging himself, his son and a lifeguard and proving that these tiny jellies caused Irukandji syndrome. They experienced severe nerve pain, muscle spasms, and vomiting. They were all admitted to hospital but survived.
Many other ocean animals use venom to subdue prey or keep themselves safe from predation. Although not all of them can take down a person, blue ringed octopuses, geography cone snails, stonefish (one of the world's deadliest fish), sea snakes, and Portuguese men of war are capable of killing humans with their venom. Meanwhile, poisonous (not venomous) pufferfish can kill you but only if you eat one.
If you think you have been stung by a harmful marine creature, call for help and get to a hospital immediately.
