8 Most Dangerous Jellyfish and 1 Stinging Imposter

The Australian box jellyfish, also known as the sea wasp, tops the list as the most dangerous jellyfish in the world. Found in the waters around northern Australia and Southeast Asia, the box jellyfish has tentacles loaded with stinging cells that deliver an incredibly potent venomous sting.

This creature stands out as not only the deadliest jellyfish, but also one of the most potent venomous species in the world.

Sea wasp venom can cause paralysis, heart failure and even death within minutes, so immediate medical treatment is crucial in the event of a sting.

This jellyfish is so dangerous that swimmers in affected areas often use nets or wear protective suits to avoid box jellyfish stings, and you should avoid waters altogether whenever large groups of this species have been spotted nearby.

The tiny but deadly Irukandji jellyfish is another serious threat, but much smaller and harder to spot than the typical box jellyfish.

Measuring only about the size of a thumbnail, this jellyfish might seem harmless, but its sting can cause Irukandji syndrome, which includes symptoms like excruciating pain, vomiting and a dangerous rise in blood pressure.

Irukandji have a widespread habitat, making themselves known in the waters of Australia, Southeast Asia, South Africa, Hawaii and even as far north as the United Kingdom. Irukandji jellyfish are responsible for multiple hospitalizations each year due to their extremely painful stings.

The lion's mane jellyfish is the largest jellyfish in the world, with tentacles that can reach up to 120 feet (36.6 meters).

This particular species is hard to miss thanks to its large size and its tendency to glow in the dark. Found in cooler waters like those of the North Atlantic and Arctic, this jellyfish delivers a sting that can cause severe pain and allergic reactions.

It's thought that human activity like fishing is lowering the number of jellyfish predators in the Arctic, causing lion's mane population to thrive. While not as deadly as the box jellyfish, the lion's mane jellyfish can still cause serious injuries, especially when swimmers are entangled in its long tentacles.

The sea nettle is another dangerous species found along the eastern coast of North America and the Gulf of Mexico. These sea nettles have long, flowing tentacles that deliver painful stings, which can cause welts, rashes and sometimes more severe allergic reactions.

While not typically deadly, the sting of a sea nettle can make swimming in affected areas an unpleasant experience.

The flower hat jellyfish is a gorgeous jellyfish species found in the coastal waters of Japan and Brazil.

But don't be fooled by its colorful appearance: This jellyfish has a powerful sting that can cause intense pain. While not typically deadly, the flower hat jellyfish is still considered dangerous due to the strength of its sting and its ability to paralyze small prey.

The moon jellyfish is one of the most common jellyfish species found around the world, including in the Atlantic, Pacific and Indian Oceans.

While their sting is mild compared to the others on this list, it can still cause irritation and discomfort, especially for swimmers with sensitive skin.

Stings can be quite frequent along touristy beaches as large numbers of these jellies wash up to the shore during high tide, but treatment is usually simple, and the pain subsides without major medical intervention.

Cannonball jellyfish, commonly found in the waters off the southeastern United States, have a less potent sting compared to other species. However, their stings can still cause irritation and mild discomfort. While these jellyfish are not deadly to humans, their venom is especially toxic to small fish.

The Australian spotted jellyfish is a large, striking species that can be found in the Indo-Pacific region. While its sting is relatively mild compared to the other species on this list, it can still cause irritation. Like the moon jellyfish, this species isn't typically a threat to humans.

However, the spotted jellyfish has recently moved to waters along the coastal United States as an invasive species, causing issues by reproducing in large numbers and eating young fish before they can develop and sustain local fisheries.

Population management of the jelly is difficult considering there can be millions of the creatures swimming in one colony.

This venomous animal is often mistaken for a jellyfish, but taxonomically speaking it isn't one. The Portuguese man o' war is actually a colony of hydrozoan organisms, but its sting is no less painful.

This species has trailing tentacles that can stretch up to 30 feet (9.1 meters) long, and despite its European name, can be found in warm tropical waters all over the world.

A sting from the Portuguese man o’ war can cause severe pain, blistering and possibly even death due to asphyxiation. Its floating, sail-like appearance makes it easy to spot, but it's best to stay far away from these creatures. If stung by a man o' war, seek medical treatment immediately.

We created this article in conjunction with AI technology, then made sure it was edited and fact-checked by a HowStuffWorks editor.