The dangerous Portuguese man o’ war, which has a potentially deadly sting, is often sighted on U.S. beaches—which leads to problems for beachgoers. Read on to learn more about these unusual creatures.
1. The Portuguese man o’ war is not a jellyfish.
The Portuguese man o’ war may look like a bloated jellyfish, but it’s actually a siphonophore—a bizarre group of animals that consist of colonies made up of dozens, hundreds, or even thousands of genetically identical individual creatures. A siphonophore starts out as a fertilized egg. But as it develops, it starts “budding” into distinct structures and organisms. These tiny organisms, called polyps or zooids, can’t survive on their own, so they merge together into a tentacled mass. They must cooperate as one in order to do things like travel and catch food.
Though the zooids within a man o’ war are basically clones, they come in different shapes and serve different purposes [PDF]. Dactylozooids are long hunting tentacles built to ensnare prey; gastrozooids are smaller tentacles which digest the food; and gonozooids are dangling entities whose job is to facilitate reproduction. Every man o’ war also has a pneumatophore, or “float”—an overgrown, bag-like polyp which acts as a giant gas bladder and sits at the top of the colony. Capable of expanding or contracting at will, it provides the man o’ war with some buoyancy control. An expanded float also enables the colony to harness winds to move around.
2. Indo-Pacific bluebottles are close relatives.
When we say Portuguese man o’ war, we’re talking about Physalia physalis, the siphonophore also known as the Atlantic Portuguese man o’ war, which can be found in warmer parts of Pacific, Caribbean, Indian, and Atlantic waters.
Another kind of siphonophore that regularly stings beachgoers is the bluebottle, Physalia utriculus. It’s sometimes called the Indo-Pacific Portuguese man o’ war and is restricted to the Pacific and Indian oceans. It’s smaller than the Atlantic species and, unlike its bigger counterpart, it hunts with a single, elongated tentacle.
3. The name Portuguese man o’ war probably refers to a naval ship.
In the Age of Sail, many European navies used tall warships loaded with cannons and propelled by the wind. British sailors took to calling this kind of vessel a “man of war.”
Physalia physalias colonies spend a lot of time floating at the water’s surface, and when the gas bladder is expanded, it looks and acts a bit like a sailboat, hence the man o’ war moniker. As for the Portuguese part, 19th-century scientists proposed that sailors encountered it near the Portuguese island of Madeira, while modern etymologists tend to think that it looked like the Portuguese version of the ship. It’s also been suggested that Renaissance-era sailors thought the pneumatophores resembled the helmets worn by Portugal’s soldiers during the 16th century.
4. Portuguese man o’ war tentacles can be up to 165 feet long.
That’s the maximum length for the dactylozooids—which are normally around 30 feet long and use venom-spewing cells to deliver painful, neurotoxic stings. When a tentacle is detached from the rest of the colony, it might wash ashore somewhere or drift around for days on end until it decomposes. Even a severed tentacle can sting you.
5. Portuguese man o’ war stings can be fatal to humans.
The odds of being killed by a Portuguese man o’ war are slim. But just because deaths are rare doesn’t mean you should touch one. In 2018, 204 people in Hollywood, Florida, were treated for stings, which can lead to red welts on the skin, muscle cramps, elevated heart rates, and vomiting.
In 1987, one victim suffered a cardiovascular collapse and died after getting too close to a man o’ war in eastern Florida. A woman swimming off Sardinia was stung by one and died of what was believed to be anaphylactic shock in 2010.
6. Some fish live in them.
Given that tiny fish make up about 70 to 90 percent of the man o’ war’s diet (it also eats shrimp and other crustaceans), Nomeus gronovii, a.k.a. the man o’ war fish, is playing a dangerous game: It lives among the siphonophore’s tentacles even though it’s not immune to its stings, swimming nimbly between the stingers. Young fish eat plankton that wanders under their hosts and, as they get older, will sometimes steal the man o’ war’s prey—or nibble on its tentacles.
7. Sea slugs steal the man o’ war’s toxins.
The man o’ war has a long list of enemies. Loggerhead sea turtles and the ocean sunfish eat them. There are also blue dragon sea slugs, which not only devour the man o’ war but actively harvest and appropriate its toxins. After storing man o’ war stinging cells in their own skins, the blue dragons can use it as a predator deterrent.
8. Portuguese man o’ wars display pretty colors.
Although it’s translucent, the float is usually tinted with blue, pink, and/or purple hues [PDF]. Beaches along the American Gulf Coast raise purple flags in order to let visitors know when groups of man o’ war (or other potentially deadly sea creatures) are at large.
9. Each colony has a distinct sex.
The man o’ war’s gonozooids have sacs that house ovaries or testes—so each colony can therefore be considered male or female. Though marine biologists aren’t completely sure how the man o’ war procreates, one theory is that the gonozooids release eggs and sperm into the open ocean, which become fertilized when they cross paths with floating eggs or sperm from other man o’ war colonies. This method of reproduction, called broadcast spawning, is also used by many species of coral, fan worms, sea anemones, and jellyfish.
10. A group of Portuguese man o’ wars is called a legion.
The man o’ war isn't always seen in isolation. Legions consisting of over 1000 colonies have been observed floating around together. Because they drift along on somewhat predictable wind and ocean currents, it’s possible to anticipate where and when a lot of the creatures will show up. For example, the Gulf Coast’s man o’ war season arrives in the winter months.
A version of this story ran in 2018; it has been updated for 2023.