The Portuguese man o' war, Physalia physalis, is a marine hydrozoan found floating in the open ocean. It has a distinct gas-filled bladder that resembles a sail, and its long, venomous tentacles can deliver painful stings. It preys on small fish and plankton, and its tentacles can extend up to 30 meters.

Physalia physalis, commonly known as the Portuguese man o' war, is native to the Atlantic, Indian, and Pacific Oceans. This species is most commonly found in warm tropical and subtropical waters, including the Gulf Stream in the North Atlantic Ocean and the Sargasso Sea. It is also seen in the Caribbean Sea and the waters off the east coast of North America, as well as around the coasts of South Africa, Australia, and New Zealand.

The Portuguese man o' war thrives in open ocean environments, typically in warm, temperate, and tropical marine waters. It prefers surface waters where it can be carried by the wind and currents across vast distances. The species is often found floating on the water's surface, relying on its gas-filled bladder, which acts like a sail, for movement.

Physalia physalis occupies a niche as a pelagic predator in marine ecosystems. It preys on small fish and planktonic organisms, capturing them with its long, venomous tentacles. The species serves as both predator and prey, providing food for certain marine animals such as loggerhead turtles and ocean sunfish, which have developed some resistance to its venom.