Meet the Very Venomous Portuguese Man-of-War

These jellyfish relatives can pack a punch, but they're also fascinating creatures.

They look like a discarded balloon. They’re actually a bubble full of agony. 

That’s why Weird Animal Question of the Week is grateful to Mishtu Som, who asked us “What is a Portuguese man-of-war exactly?”  

You’ll want the skinny on these dangerous beauties if you encounter one on your summer beach day. 

Out of Many, One

Named for the buoyant sail that resembles an ancient ship, these creatures should be called "men-of-wars," because they’re not one animal, but many.  

A man-of-war is a colony of hydroids, tiny animals which “work together to form a total package,” says George Burgess of the Florida Museum of Natural History.  (See "Deadly Beauty: A Portrait of the Portuguese Man-of-War.") 

Therefore, they are not jellyfish, though they strongly resemble their cousins of the sea. 

“Johnny Cash had a song about working in an auto factory,” stealing parts over years to build one Cadillac, Burgess says. “That’s what a Portuguese man-o-war is. A 'One Piece at a Time' organism.”  

Unlike that Caddy, though, they have no propulsion system and depend on currents and wind for mobility in warm tropical and subtropical waters worldwide. 

For instance, man-of-wars near Florida can get blown far north by the Atlantic Ocean's Gulf Stream, Quinton White, a marine biologist at Jacksonville University, says via email.  

Sting Operation

Man-of-wars have biological structures called polyps that serve various functions such as feeding, defense, and reproduction. 

One polyp that acts like an air bladder is filled with gases—including carbon monoxide, oxygen, and argon—that keeps the animal afloat.  

If attacked by birds, the man-of-war also has a siphon that allows the gas to leak from their air bladder and then escape underwater, Burgess says. (See "The Surprising Way Jellyfish Put Themselves Back Together.") 

(Kind of like if your inflatable pool floats could get away from you. And were venomous.) 

Man-o-war tentacles can reach 165 feet (50 meters) long and are covered with nematocysts—“coiled, barbed bodies” that look like a squished Slinky, Burgess says. (See pictures of colorful sea creatures.) 

On contact with prey, such as small fish, “boing! Out they come,” he says—along with an injection of disabling poison. Other parts of the tentacle pull the prey upward toward the mouth, where an injected enzyme starts the digestive process. 

Saved by the Bell

One fish has hacked the system. The Portuguese man-of-war fish hangs out under the predator's "float" (called a bell), where there are fewer stingers, nibbling on its host's tentacles and “nutrient-rich reproductive organs." 

The man-of-war has predators, too. Ocean sunfish, with their leathery skin, and loggerhead sea turtles, whose “mouths are hard as nails,” can chow down on them without getting stung, Burgess says.  

For us humans, though, a man-of-war sting can cause welts and severe pain—and allergic reaction can affect a person's breathing, White says. "Drowning is a concern."  

Burgess, an asthmatic, needed hospital treatment after a sting and advises simple caution in the ocean. 

“Live your life,” he says, but “remember when you enter the sea that it’s a wilderness experience.”  

Weird Animal Question of the Week answers your questions every Saturday. If you have a question about the weird and wild animal world, tweet me, leave me a note or photo in the comments below, or find me on Facebook.

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