Jellyfish can be fascinating and beautiful, but for most people, it’s the potentially deadly sting they’re concerned with.
There are literally thousands of known jellyfish species in the world’s oceans, and most species don’t sting humans. However, we’re here to look at the dangerous jellyfish and find out what jellyfish can kill you.
The most dangerous jellyfish in the world is the Australian box jellyfish. This particular box jellyfish has stinging cells so deadly they could potentially kill a human in just a few minutes.
We’re going to take a deep look at the deadliest jellyfish in the world and then explore nine others that you definitely don’t want to meet while taking a swim.
What Is the Most Dangerous and Deadliest Jellyfish?
The most dangerous and deadliest jellyfish is the Australian box jellyfish (Chironex Fleckeri), also known locally as the sea wasp.
It’s found in coastal waters from northern Australia and New Guinea, and those as far north as Thailand, Malaysia, the Philippines, and Vietnam.
Why is the box jellyfish so dangerous? Because it has the worst jellyfish sting of all.
The sting is said to be so strong that if you came into contact with enough of the venom-loaded cnidocyte cells, you could die in between two and five minutes.
Scientists say one sea wasp can theoretically carry enough venom to kill at least 60 adult humans.
What Happens if They Sting You?
If you come into contact with a sea wasp’s tentacles, you’ll know about it immediately. The stinger cells fire their venom-filled darts into your skin, and you’ll feel extreme pain, similar to being badly burnt.
Getting quick first aid and then rapid emergency medical attention for box jellyfish stings is vital.
The recommended first aid in Australia is to immediately pour large amounts of vinegar onto the wound while waiting for an ambulance. The vinegar deactivates undischarged nematocysts and might stop the sting from getting worse.
A sea wasp victim should never be left alone as they could become unconscious and need CPR.
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It’s even known that swimmers become unconscious and drown after they’ve been badly stung before they could make it back to shore.
Can You Survive a Box Jellyfish Sting?
There have been more than 64 fatalities in Australia since records began in 1883. However, fortunately, most people only make slight contact with the tentacles and suffer milder symptoms.
If you get emergency first aid and emergency care quickly, the chances of survival are pretty good. Emergency personnel can give anti-venom for box jellyfish stings, which may dramatically improve the chances of a full recovery, even in the case of nasty stings.
The most deadly jellyfish is definitely one you want to avoid, so if you’re in the areas of the world where sea wasps occur, you should always take note of local warnings and only swim where it’s recommended.
There’ll usually be nets in popular locations to keep jellyfish out, but you should keep your eyes open.
What Is the 2nd Most Dangerous Jellyfish?
The second most dangerous jelly is the four-handed box jellyfish (Chiropsalmus quadrumanus).
It might not be as venomous as the Australian box jelly. However, this box jelly has a more extensive distribution worldwide. So, you might be more likely to encounter it.
The four-handed box jellyfish can be found in warmer waters of the western Atlantic Ocean, the Gulf of Mexico, and the Pacific Ocean, including around Hawaii and Australia.
On the eastern coast of the United States, this box jellyfish has been responsible for stings in waters off North and South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, and Texas.
The sting from the four-handed box jellyfish is similar to the sea wasp, and the first aid recommendations are the same.
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Intense pain can be followed by cardiac and respiratory dysfunction, depression, or arrest. Assuming recovery from more dangerous symptoms, victims will experience a painful rash for several months.
These box jellyfish have killed several people, and unfortunately, children appear especially vulnerable to dying after a sting due to their lower body weights.
Top 10 Deadliest Jellyfish
Can a jellyfish kill you? Yes. It could be your time if you meet the wrong jellyfish and don’t get emergency care.
Even the less dangerous jellyfish can cause extreme pain and could cause fatalities in children, older people, or anyone with a medical condition.
Here are the jellyfish you should look out for to stay safe at the beach or in the ocean.
1. Deadly Box Jelly (Class Cubozoa)
There are at least 50 species of box jellyfish, and in addition to the deadly Australian box and four-handed jellyfish, several others have been known to cause injuries and human deaths.
Examples of the deadliest jellyfish include the Viper box jellyfish (Chironex yamaguchi) found in Japan and less well-known examples without common names like Chiropsoides buitendijki, and Chirodropus gorilla. These can all deliver a potent venom leading to severe pain and maybe worse.
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However, as this collection of deadly jellyfish all exhibit more or less the same characteristics, we can band them together.
Cubozoa is the smallest class of phylum cnidaria. The most apparent shared characteristic of all the species of the box jellyfish family is their box-like bell.
The box jelly pulsates its bell to make a powerful water jet, enabling many species to swim quite quickly. Scientists have measured an Australian box jelly swimming as fast as 6.9 meters (22.6 ft) per minute.
Long, slender tentacles covered in stinging cells called nematocysts hang from each of the four lower corners of the jellies bell. It’s these which have gained the box jellyfish its reputation as “the world’s most venomous creature.”
Box jellyfish can be found throughout the world’s subtropical and tropical oceans. However, the most deadly are generally limited to the tropical Indo-Pacific area.
It’s thought that the box jellyfish is more intelligent than most other jellyfish and possesses more advanced senses.
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For example, larger box jellyfish have four fully developed eyes that can distinguish points of light and twenty simple light and dark detecting eyes. These give them a highly advanced ability to see their prey and avoid predators and obstacles.
Box jellyfish will use their sight and swimming abilities to hunt small fish and crustaceans to feed on.
2. Irukandji Jellyfish
Is there a jellyfish that makes you suicidal? Yes! The Irukandji jellyfish has been called the “doom jellyfish” because suicidal thoughts of impending doom have been reported after victims have been stung.
Irukandji jellyfish are tiny box jellies measuring just a cubic centimeter or so when fully grown. However, these species of box jellyfish pack a punch far greater than their size.
Irukandji syndrome, the name given to the condition when people have been stung, often unknowingly, can be extremely painful and has even caused a few human deaths.
Thanks to their tiny size and see-through body, the Irukandji are extremely hard to see. It’s often the case that a swimmer doesn’t know what stung them.
At first, the box jellyfish stinging cells may feel relatively mild. However, after about 30 minutes, Irukandji syndrome can develop a range of nasty symptoms, including severe pain across all body areas, nausea, vomiting, sweating, and anxiety.
The cardiac and respiratory systems can act up, resulting in high blood pressure, an irregular or rapid heartbeat, and fluid in the lungs.
In most cases, an irukandji syndrome patient improves with medical attention after about a day. However, symptoms can last up to two weeks.
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Irukandji jellyfish are native to the oceans of northern Australia. However, it is believed they may have spread to Florida, Japan, and Britain, perhaps carried in ships’ ballast water.
3. The Fire Jelly (Morbakka Fenneri)
The fire jellyfish is another smaller species of box jelly, and if you get stung by it, the symptoms are identical to the dangerous Irukandji syndrome.
However, unlike the tiny Irukandji we’ve already talked about, this potentially deadly species of jellyfish has not only been observed and studied by scientists in Northern Australia. They also occur in Thailand.
Fire jellyfish are also larger than the Irukandji jellyfish, with a bell that can reach between 6 and 18 centimeters (2.3 to 7 inches) long.
The stinging tentacles can reach up to 50 cm (19.6 inches) in length and are mauve colored.
These jellyfish have become more and more common in recent years. So they’re definitely worth being aware of if you visit the affected areas.
4. Nomura’s Jellyfish (Nemopilema Nomurai)
Our next potentially deadly jellyfish is one of the largest species of jellyfish found anywhere.
Nomura’s jelly is a rhizostome jellyfish found in the seas between China and Japan that can grow as large as 2 meters (6 ft 7 in) in diameter and weigh over 200 kg (440 lb).
To go with its incredible size, the Noumra jelly also has a potent sting that, in severe cases, can be deadly.
Anyone stung will experience, at a minimum, extreme pain, itching, inflammation, and reddening skin.
Due to the complex nature of the venom’s proteins, there isn’t an anti-venom available for Nomura stings. Anyone stung should immediately rinse the affected area with seawater and seek emergency medical attention.
Unlike box jellies, you shouldn’t use vinegar as scientists have found it “immediately caused significant nematocyst discharge,” which could worsen the situation.
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5. The Sea Wasp (Alatina Alata)
Although they share a commonly used name, this “sea wasp” box jellyfish isn’t nearly as dangerous as the Australian box jelly at the top of our list.
In fact, there are no reported deaths on record caused by this box jellyfish species.
However, this sea wasp found in the warm waters of the Atlantic, Indian, and Pacific Oceans, as well as in the Arabian and Caribbean seas, can still cause severe pain, making it a worthy addition to our most dangerous list.
Most people find that the sting’s painful and itchy rash goes away after about a day. However, some sensitive victims require medical attention to deal with the discomfort and potentially, although rare, complications.
6. Sea Nettles (Chrysaora)
There are 16 species in the genus Chrysaora, and they typically all get called sea nettles in the various habitats they’re found in.
All species, including the black sea nettle (Chrysaora achlyos), have a painful sting but are not usually deadly unless the unfortunate victim experiences a severe allergic reaction.
When the nematocysts inject venom into human skin, the sting and inflammation usually last for about an hour.
You’ll find the black sea nettle between Monterey Bay and Baja California in the Pacific Ocean. This is a giant jellyfish with a bell reaching up to 1 meter (3 ft) in diameter and tentacles stretching as much as 5 or 6 meters (16 or 20 ft).
Chrysaora sea nettles are found in almost all the planet’s seas, including tropical and subtropical oceans, the North and South Atlantic Ocean, the Central Info-Pacific Region, and the North and South Pacific Ocean.
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7. Cannonball Jellyfish, AKA Cabbagehead Jellyfish (Stomolophus Meleagris)
Cannonball jellyfish are found in the Pacific, the Atlantic, and the Gulf of Mexico. They’re most common on the Atlantic coast from the United States to Brazil.
In summer and fall, cannonball sightings can be widespread. In fact, the cannonball is responsible for most of the reported jellyfish stings in the whole of the United States and the Caribbean.
The cannonball delivers a stinging mucus into the water from its nematocysts which can occasionally cause cardiac arrest in humans if there’s enough contact or ingestion.
The mucus is also extremely painful if it gets in your eyes, although it’s generally not harmful in the long term.
Most people get away with an itchy rash. However, giving a cannonball a wide berth is undoubtedly a good idea if you see one, as you don’t need to touch it to feel the effects.
8. Lion’s Mane Jellyfish (Cyanea capillata)
The lion’s mane jellyfish is a giant species found in cold waters, including the Arctic, northern Atlantic, and northern Pacific, so you’re probably unlikely to meet it while swimming.
That’s probably a good thing as this jelly’s tentacles can reach as far as 36.6 m (120 ft)!
While most stings are only painful, if you come into contact with enough of the tentacles, you could be looking at a trip to the hospital.
Severe stings have been thought to cause drowning when the victim experiences panic caused by the pain.
9. The Mauve Stinger (Pelagia noctiluca)
Mauve stingers are found in tropical and warm temperate waters throughout the world.
As well as their distinctive mauve color, the stinger is known for appearing in large numbers, especially in the Mediterranean Sea.
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Hundreds of jellyfish get washed up onto beaches, and it’s worth knowing that even dead jellyfish can still deliver their venom if handled.
Stings from the mauve stinger are common for summer swimmers and are often painful for up to two weeks. As well as the burning pain, unlucky victims experience a swollen rash and often dizziness, nausea, and diarrhea.
10. A Bonus Entry – The Portuguese Man o’ War (Physalia physalis) – Not a Jellyfish!
Our final entry for the most dangerous jellyfish isn’t actually a jellyfish at all.
However, most people think that the Portuguese Man o’ War is a jellyfish, and it has a potentially deadly sting, so it’s worth a mention.
The Portuguese man o’ war is something called a marine hydrozoan. They have an unusual translucent blue, purple, pink, or mauve-colored gas-filled sack that floats on the surface and acts like a sail.
Below the water, there are huge tentacles that can be as long as 10 and 30 meters (30 to 100 feet.)
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The sting from the tentacles is one of the worst out there. A Portuguese man o’ war sting can cause breathing and cardiac difficulties, fever, shock, and even death if left untreated, so urgent emergency medical attention is always needed.
What’s the Least Dangerous Jellyfish Species? – The Moon Jellyfish (Aurelia aurita)
The moon jellyfish is probably the least dangerous jellyfish species, and it’s also the most common in oceans worldwide.
Moon jellyfish are relatively flat and have purple-colored translucent bodies. There are tiny tentacles underneath, but these are used for trapping plankton, and unless you’re incredibly sensitive, you probably won’t feel a thing if you touch one.
The biggest threat the moon jelly can cause is irritation at how many there can be. They’re common in tropical seas and can often build up in considerable numbers in calm bays where people enjoy swimming.
Although so many jellyfish can look threatening, most people can happily push them out of the way without concern.
The most dangerous jellyfish is undoubtedly the Australian box jellyfish. Along with other species of box jellyfish, they can cause severe medical problems and even death if you come into contact with one.
Fortunately, considering how many people swim in the ocean each year, venomous jellyfish incidents are infrequent.
However, you should always follow local guidelines, wear protective stinger suits, or swim in net-protected areas if appropriate.
British-born Dan has been a scuba instructor and guide in Egypt's Red Sea since 2010.
Dan loves inspiring safe, fun, and environmentally responsible diving and particularly enjoys the opportunity to dive with sharks or investigate local shipwrecks.
When not spending time underwater, Dan can usually be found biking and hiking in Sharm's desert surroundings.