The Portuguese Man o' War, "Physalia physalis", is a large colonial marine jellyfish. It is composed of many tiny individual zooids, which are specialised polyps and medusae. The colony has an air bladder, and floats on the surface. It delivers a massive sting to its prey. Humans, if stung, may require medical attention. It is often accompanied by fish which are immune to the stings. This type of hydrozoan is called a siphonophore.