If you had to pick a sea creature that you didn’t want to get into the water with, jellyfish might be up there in your top picks, says Melissa Hobson. In every one of the world’s oceans, these gelatinous invertebrates drift through the water with their tentacles stretching behind them like a bride’s veil.
But if you happen to touch these dangling appendages, you might be in for a nasty surprise. Jellyfish tentacles can deliver a venomous sting – and they can still hurt you when they're dead.
There are over 2,000 different jellyfish species (that we know of) and most are harmless to humans. But which are the deadliest? Here are some of the jellies considered most dangerous to humans.
Deadliest jellyfish
Australian box jellyfish
(Chironex fleckeri)
There are around 50 different species of box jellyfish, which are sometimes known as ‘sea wasps’. Just eight of these are known to be able to kill humans.
The deadliest of all is the Australian box jellyfish (Chironex fleckeri), which is believed to be the most deadliest creature in the ocean. Its tentacles contain a toxin so potent that a severe sting can lead to painful lesions, headaches, anxiety, vomiting, breathlessness, a spike in blood pressure, a ‘sense of impending doom’ and even cardiac arrest. Death can come within five minutes.
The Irukandji jellyfish
(Malo spp.)
As we know from the teeny blue-ringed octopus, size isn’t everything when it comes to venom potency. This is also true of Irukandji jellyfish – species in the Malo genus that can be as small as one centimetre. But, like the box jelly, these tiny terrors can be deadly if you get on the wrong side of them.
Their sting can cause Irukandji syndrome, which can be fatal. Symptoms include severe pain, cramping, headaches, high blood pressure, coughing, and vomiting.
Barnes’ jellyfish
(Carukia barnesi)
Known to cause Irukandji syndrome, this species is sometimes also referred to as an Irukandji jellyfish. It gets the name ‘Barnes’ from a scientist called Jack Barnes who discovered the link between this little critter (which is around the size of a peanut) and Irukandji syndrome.
He must have been very dedicated to his work as he solved the mystery by stinging himself, his son, and a lifeguard to see what happened. Unfortunately for them, he was right and they experienced Irukandji syndrome. Thankfully, after being rushed to hospital for treatment, all three survived.
Winged box jellyfish
(Alatina alata)
Winged box jellyfish are another species whose sting can cause cardiac arrest and prove fatal. When scientists studied the toxins in the sting, they found it contains a gene that can break down red blood cells. Yikes.
Four-handed box jellyfish
(Chiropsalmus Quadrigatus)
Found in tropical Pacific waters, this type of box jelly has a less severe sting than other box jellyfish species but it can leave scarring and can still be fatal in some instances. It is officially recorded as killing at least three people off the coast of Japan.
Portuguese Man o’ War
(Physalia physalis)
Although people often call the Portuguese man of war a jellyfish, it’s actually a siphonophore. It is made up of a colony of thousands of organisms that work together as if they were one animal – some are responsible for feeding, some for reproduction, some for defence and some make up the ‘sail’ on top which helps the Man o’War travel across the ocean.
Stings are very painful and often have similar symptoms to those of a jellyfish. It’s rare for its sting to kill a human but it has happened.
Lion's mane jellyfish
(Cyanea capillata)
For many years, the lion’s mane jelly was thought to be the biggest jellyfish in the world because of an 1856 claim that a geologist caught one measuring 2.3 metres in diameter with 37-metre tentacles. Although many now believe this was an exaggeration, it’s still among the largest species of jelly.
Named for its bushy red tentacles (it has over a thousand of them), which resemble a lion’s mane, this huge jellyfish can inflict a painful sting. If you touch them, expect inflammation and welts to develop on your skin. As well as the pain and swelling, symptoms can include headaches, nausea and muscle pain.
Despite what Sir Arthur Conan Doyle might have us believe (in the 59th Sherlock Holmes story, The Adventure of the Lion's Mane, this jellyfish caused the death of a schoolteacher), fatalities from Lion’s mane stings are incredibly rare. If you do get on the wrong side of these trailing tentacles, it’s best to seek medical advice as soon as possible in case you have an allergic reaction.
Nomura's jellyfish
(Nemopilema nomurai)
With a giant bell measuring two metres in diameter, and tipping the scales at up to 200 kilograms, the Nomura's jellyfish's believed to be the largest and heaviest species of jellyfish.
Its sting can also be incredibly dangerous, causing pain, swelling, shock and – sometimes – death. Researchers wanted to find out which of the toxins in its venom caused death so they caught some specimens, collected and separated the toxins and then tested each one on mice to see which would cause death (harsh). In the end, they realised that it was probably a combination of all the toxins together that had the lethal effect.
Sea nettle
(Chrysaora quinquecirrha)
As the name suggests, a brush with a sea nettle might leave you with a painful souvenir. Although their sting is not deadly, unless you have an allergy, it can cause pain and irritation.
Upside-down jellyfish
(Cassiopea sp.)
The upside-down jellyfish is a curious creature that likes to rest upside down on the floor – hence its name. Although they are known to sting swimmers – and it can cause severe pain – they aren’t typically lethal.
