Portuguese man o' war

The Portuguese man o' war (Physalia physalis), also known as the man-of-war or bluebottle, is a marine hydrozoan found in the Atlantic Ocean and the Indian Ocean. It is the only species in the genus Physalia, which in turn is the only genus in the family Physaliidae. The name man o’ war comes from the man-of-war, a sailing warship, and the animal's resemblance to the Portuguese version (the caravel) at full sail.

The man o' war is part of the neuston, organisms that live on the surface of the water. A gas-filled bladder 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, and even, in some cases, humans. A sail on the bladder, which may be left or right-handed, propels it about the sea, often in groups. 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.

Because it frequently washes up on beaches on the coast of the Atlantic, Indian and Pacific Oceans and is often seen in the open ocean, the man o' war is the most well-known siphonophore, as nearly all siphonophores live in the cold, dark ocean depths where they can only be observed in their natural habitat by a submersible or ROV.