The Portuguese man o’ war is not a jellyfish but a siphonophore, a colony of zooids that all grow from one fertilized egg and share identical DNA. Through gene expression each zooid locks into a job: dactylozooids hunt and defend, gastrozooids digest, and gonozooids reproduce. Its float, the pneumatophore, can reach 30 centimeters and holds air plus up to 13 percent carbon monoxide made by a gas gland, letting it inflate, or deflate to sink away from threats. The inflated float resembles a Portuguese caravel, the source of the name. With no fins or motor, it drifts with wind and current and trails tentacles usually about 10 meters long and sometimes up to 30, armed with spring loaded nematocysts that paralyze fish almost instantly before the catch is reeled up to the gastrozooids.

In Australia the colonies cause up to 10,000 human stings each summer, raising whip-like welts and pain lasting one to three hours, with rare deaths mostly among small children. Beached specimens and loose tentacles can sting for days. Vinegar locks unfired nematocysts, and immersion in 45 degree Celsius water for 20 minutes breaks down the venom proteins. Predators have found workarounds: loggerhead turtles and ocean sunfish have tough mouths, the blue dragon sea slug stores stolen nematocysts, and blanket octopuses carry torn off tentacles as weapons. Each colony is left-handed or right-handed and sails about 45 degrees off the wind, so a storm can beach at most half a population. Colonies are all male or all female, and their spawning has never been observed in the wild.

• The man-of-war fish lives among the tentacles and may lure prey for the colony.
• The violet sea snail floats on a raft of mucus bubbles to hunt the man o’ war.
• Researchers run drift models backward from beachings in Ireland and the Bay of Biscay to trace origins.
• Genetic work splits the old single species into Physalia physalis, Physalia utriculus, and Physalia minuta.
• A new species, Physalia mikazuki, was described off Japan in 2025.