The Portuguese man-of-war, also known as the bluebottle, is not really a jellyfish, as commonly believed. It doesn't swim like a jellyfish - it floats on top of the sea, made buoyant by a gas-filled sac or "bottle" that gives it its alternate name. It is related to corals and sea anemones and is composed of four different types of polyps - float, tentacles, and digestive and reproductive polyps. The float is typically less than a foot long, but the tentacles extend many feet into the water. Part of the float is formed into a crest, which catches the wind as a sail and moves the animal through the water. Some Portuguese men-of-war are oriented to sail toward the right and others toward the left, which helps to disperse the species more widely throughout the ocean.

Common in tropical waters, the Portuguese man-of-war is often seen off the coasts of Hawaii, Australia, and the Florida Keys. Its sting is not usually fatal, but can be extremely painful. Some people have severe reactions to the sting and go into shock. Tentacles will often stick in the wound and need to be removed, which can easily cause further injury if not done carefully.