The Portuguese man o' war is a highly venomous, free-floating marine cnidarian.

The Portuguese man o' war is a floating cnidarian that lives in warm seas and belongs to the siphonophores. Although its name contains the word "jellyfish," it is not a true jellyfish in the strict sense, but rather a colony organism made up of many individuals with specialized functions; these individuals divide the work and cooperate to keep the whole colony alive. Its most striking feature is the translucent blue-purple float at the top, shaped like a monk's hat and also like a small sail, which lets it drift across the sea surface with the help of sea winds and ocean currents. Below the float hang long tentacles, which can reach several meters or even longer, densely covered with stinging cells that are its main weapon for catching small fish and defending against enemies. The Portuguese man o' war's venom is quite notorious among floating organisms; after a human is stung, reactions such as burning pain and redness and swelling appear, and medical attention should be sought promptly if symptoms are severe. It should be noted that even if a Portuguese man o' war washed up on the beach is already dead or has broken apart, its stinging cells may still remain active, so when visiting the seaside, never touch it with bare hands or poke at it with your feet. In the marine ecosystem, the Portuguese man o' war also has its own natural enemies; animals such as sea turtles have a strong tolerance for its venom and can use it as food. Its way of life, drifting with the waves, not only helps it evade most enemies, but the shade beneath its tentacles also often attracts small fish to gather, forming a unique symbiotic relationship.