Largest siphonophore zooids

The longest siphonophore zooids are the stinging, prey-capturing tentacles, or dactylozooids, of the Portuguese man o' war (Physalia physalis), each of which is a single multicellular organism in its own right and can attain a total length of up to 50 m (164 ft), hanging vertically down through the water. This species occurs floating at the water surface of subtropical and tropical oceans globally if, as many experts now believe, the Pacific man o' war or blue bottle (P. utriculus) is conspecific with it.

Despite superficially resembling true jellyfishes (scyphozoans), each specimen of this species is actually a colony of intimately associated multicellular organisms called zooids. These consist of four different morphological types: the dactylozooids, or stinging tentacles; the gastrozooids, or feeding tentacles; the gonozooids, or reproductive polyps; and the single gas-filled floating pneumatophore, or float polyp.

The Portuguese man o' war also holds the less desirable accolade of being deemed the most dangerous siphonophore, because it is the only known species to have been responsible for human death, as a result of its stinging, prey-capturing tentacles' deadly potency if touched, even when beached. The first confirmed human fatality occurred on Florida's Atlantic coast in 1987.