The Most Dangerous Jellyfish On The Planet

Jellyfish — or sea jellies, as many marine scientists prefer to call these non-fishes — have thrived for over 500 million years. This means they've lived through every single major extinction event in Earth's history and are among the 1% of species that didn't get wiped out (via Scientific American). This also means that jellyfish are the oldest group of multicellular animals around, as jellyfish expert David J. Albert explained in an interview with The New York Times. That's a pretty mean feat, considering how they're literally brainless bags of water.

Most people don't think about any of this when they hear the word "jellyfish," though. Instead, the first thing that comes to mind is their sting. A 2006 paper places the total number of jellyfish stings per year at an estimated 150 million. Admittedly, this is challenging to pin down, since a good number of jellyfish sting incidents go unreported (via Science). And while not all jellyfish species sting, the ones that do can turn a trip to the beach into a total disaster, especially the ones on this list. (Oh, and in case you were wondering, while it belongs to the same phylum as jellyfish, the infamous Portuguese man-o-war is actually a siphonophore — a colony of single-celled organisms operating as a single individual — and is therefore ineligible here.)

Certain types of jellyfish such as the moon jellyfish (otherwise known as, appropriately enough, the common jellyfish) are easy enough to find in their known habitats, giving scientists more than enough opportunities to study them. And then there's the stinging cauliflower, a species of jellyfish so rare, there have only been a small handful of confirmed sightings since its discovery in 1880 (via the University of Cyprus Oceanography Center).

Wondering why this species is such a rare sight? Scientists aren't completely sure yet, although Instituto Oikos suggests that it's because it spends most of its time as a polyp. In this stalk-like phase, the animal attaches itself to a coastal reef, nourishing itself until it reaches adulthood and floats around as a full-grown jellyfish (via Te Ara). Confirmed sightings were reported in 1945 and 2014. Some claimed to have also seen it in 2000, 2020 (according to Sputnik News), and 2021 (via the Gibraltar Chronicle).

Ranging in color from pale rose to golden brown, the stinging cauliflower's bell grows up to 3 feet in size, according to the Gulf Coast Research Laboratory. It has about 150 long tentacles and is even known to feed on other jellyfish. Regarded as "highly venomous," experts say its venom can have devastating effects on humans. In other words, it's good this jellyfish isn't seen often — and appreciated (from a distance) when it is.

Imagine this — you're snorkeling in North Queensland, admiring the Great Barrier Reef. The abundance of colorful life distracts you so much that you don't even notice a tiny jellyfish, no more than an inch and a half in size, drifting towards you. The sting barely registers on your hand, and you quickly forget about it as you swim back up to the surface. Your true ordeal starts 30 minutes later — all at once, you experience a gigantic wave of pain in the form of simultaneous nausea, headaches, intense sweating, joint pain, high blood pressure, irregular heartbeat, and a bunch of other nasty effects. As your companions apply first aid and prepare to take you to the nearest hospital, you silently wonder through the pain, "How can something so small bring you so close to death's door?"

Meet Carukia barnesi, one of a small number of sea jellies sharing the common name "Irukandji jellyfish" (according to a 2006 study in QJM). All Irukandji jellyfish can cause what experts call the Irukandji syndrome, essentially a smorgasbord of serious symptoms like the ones mentioned above. There have even been two reported deaths, both of which died due to a hemorrhage within the skull (as cited by a paper published in 2020). Worse, there is currently no antidote for Irukandji jellyfish venom. Despite the fact that the death rate is relatively low, you're unlikely to ever forget the experience of getting stung by this highly dangerous jellyfish.

The "kingslayer" jellyfish bears its name for an important — and tragic — reason — its discovery in 2007 was the direct consequence of its venom killing an American tourist named Robert King. It's classified under the Malo genus just like Malo maxima, which means it's yet another jellyfish capable of causing Irukandji syndrome. However, unlike its fellow Malo jellyfish, the common kingslayer has four tentacles encircled by tissue matter resembling a halo.

The common kingslayer has been observed to grow up to about an inch and a quarter. As with most Irukandji jellyfish, its sting, while excruciatingly painful, rarely causes death. Upon contact, it shoots its venom into the victim's bloodstream via its dart-like nematocysts.

Interestingly, the Australian jellyfish expert who described the common kingslayer, Lisa-Anne Gershwin, also had a close encounter with this tiny yet terrifyingly dangerous animal. According to Scientific American, the only thing that saved her life was the fact that the jellyfish that stung her was most likely just a juvenile.

A common thread among many of these jellyfish is that as long as you stay away from them, you won't feel their sting. Unfortunately, Mother Nature enjoys occasionally pulling a fast one, which explains why she gave the ability to weaponize water to such a ridiculous-looking animal — the upside-down jellyfish, named after its tendency to rest upside-down at the bottom of Australia's coastal lakes, lagoons, estuaries, and mangrove forest waters (via the Australian Museum).

The upside-down jellyfish has eight mouth arms that house algae called zooxanthellae. These organisms give the jellyfish's appendages their greenish, grayish, or bluish tint. Their relationship with the jellyfish is mutually beneficial — while its tissue serves as protection for the algae, they sustain their living home by providing it with nearly all of its nutritional requirements.

As mentioned earlier, this jellyfish has the unique ability to turn its shallow, typically meter-deep surroundings into "stinging water." There have been reports of humans emerging from an otherwise normal swim with rashes all over, according to Inside Science. A study published in 2020 in Communications Biology revealed how the upside-down jellyfish does this — through the use of cassiosomes, stinging-cell structures (or "mobile grenades," as the researchers called them) that it releases into the water when it expels mucus, often as a precautionary measure against potential threats.

Honestly, all that one would need to know about the aptly named "sea wasp" is that it can kill a person in less than five minutes (via the Northern Territory Government of Australia). Widely regarded as the most deadly jellyfish in the world, this incredibly dangerous box jellyfish species has been responsible for about 100 deaths in Australia over the last century, according to the University of California Museum of Paleontology.

The sea wasp is the largest known box jellyfish, with its body growing up to nearly 12 inches in size and weighing a little over 13 pounds (via the University of Melbourne's School of Biomedical Sciences). Despite having up to 60 tentacles measuring almost six-and-a-half feet long, its crystal-clear body underwater makes the sea wasp challenging to spot for swimmers and divers in northern Australian waters. The sea wasp delivers its powerful venom upon skin contact through its tiny nematocysts. A single tentacle reportedly contains "many millions" of these harpoon-like stinging cells.

If you're unfortunate enough to be stung by a sea wasp, you will experience hot, searing pain within a short period of time, quickly followed by skin welting and possibly permanent scarring. In severe stinging cases, the victim may suffer a heart attack after a few minutes, which is the usual cause of death in such incidents. As children have a smaller body mass, sea wasp stings tend to be particularly deadly for them.

Notorious among fishermen and divers in Japan for its "fiery sting," the fire jellyfish is one of the largest. The sensory structures within its elongated bell resemble rabbit ears, which is unfortunately the only remotely adorable thing one can say about this sea jelly.

The fire jellyfish grows up to almost 10 inches in height and reaches a maximum width of about 8 inches. Its rectangular, warty body possesses tentacles that grow up to nearly 10-feet long. At night, it typically uses its envenomed sting to catch prey, such as the Japanese anchovy, according to a 2018 analysis of its stomach contents.

Some experts believe that the fire jellyfish's sting can cause the dreaded Irukandji syndrome, which means its victims may suffer a litany of serious symptoms before succumbing to a heart attack. Still, more research on this dangerous jellyfish is required before scientists can determine the full extent of its toxicity.