The Portuguese man o' war (Physalia physalis), also known as the man-of-war or bluebottle, is a marine hydrozoan found in the Atlantic, Indian, and Pacific oceans. While it is typically considered the only species in its genus, Physalia, and family, Physaliidae, genetic evidence suggests there may be more.

Although it superficially resembles a jellyfish, the Portuguese man o' war is in fact a siphonophore. Like all siphonophores, it is a colonial organism, made up of many smaller units called zooids. Although they are morphologically quite different, all of the zooids in a single specimen are genetically identical. These different types of zooids fulfill specialized functions, such as hunting, digestion and reproduction, and together they allow the colony to operate as a single individual.

The man o' war is part of the neuston, organisms that live on the surface of the water. A gas-filled bladder called the pneumatophore provides buoyancy that lets the animal stay afloat on the surface of the water while its tentacles, which can be up to 30 m (100 ft) long, hang below the surface, containing venomous cnidocytes that help capture prey. The cnidocytes can deliver a sting powerful enough to kill fish, crustaceans, and in some cases, humans. A sail on the pneumatophore propels it about the sea, sometimes in groups as large as 1,000 individuals. The sail may be left or right-handed, based on what direction the wind catches it.

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