The Portuguese man o’ war (Physalia physalis), also known as the man-of-war, bluebottle, or floating terror is a marine hydrozoan found in the Atlantic Ocean and Indian Oceans. It is considered to be the same species as the Pacific man o’ war, which is found mainly in the Pacific Ocean.
The Portuguese man o’ war is the only species in the genus Physalia, which in turn is the only genus in the family Physaliidae. It has numerous venomous microscopic nematocysts which deliver a painful sting powerful enough to kill fish, and has been known to occasionally kill humans. Despite its appearance, the Portuguese man o’ war differs from single organisms like jellyfish as they are siphonophores, a colonial organism made up of many specialized, though genetically distinct, parts called zooids. These zooids are attached to one another and are physiologically integrated to such an extent that they cannot survive independently. The assemblage of zooids works together to function as an individual animal. Zooids should not be confused with zooplankton.