Portuguese man o' war

The Portuguese man o' war ("Physalia physalis") is a Cnidarian invertebrate. It lives in the sea. It is sometimes called the "blue bubble", or "man-of-war". Its common name comes from a Portuguese war ship of the 15th and 16th century. The ship had triangular sails, similar in shape to the bladder of the man o' war.
The Portuguese Man o' War has a reputation for swarming in groups of thousands. It is also known for its powerful sting.
Structure.
"Physalia" is not a single animal: it is a siphonophore. This is a colony of four kinds of zooids. Zooids are very small, highly modified individuals. These zooids are specialized polyps and medusoids. Though structurally similar to other cnidarians, the zooids do not live by themselves: they are attached to each other. Each type of zooid does not have some structure and functions. It depends for survival on the others doing what it cannot do by itself.
A similar group of animals are the chondrophores. They are specialized hydroids, which also float at the surface of the open ocean.
Predators.
The loggerhead turtle feeds on the Portuguese man o' war. It is a common part of the turtle's diet. The skin of the turtle is too thick for the man o' war's sting to enter and launch its venom. The sea slug "Glaucus atlanticus" and the violet snail "Janthina janthina" also feed on the Portuguese man o' war. The blanket octopus "Tremoctopus" is immune to the venom of the Portuguese man o' war. The blanket octopus has been known to rip off the man o' war's tentacles and use them for defensive purposes.
Commensalism and symbiosis.
The Portuguese man o' war is often found with many different kinds of marine fish. Some of them are shepherd fish, clown fish, and yellow jack. These species are rarely found anywhere else. The clown fish can swim among the tentacles without being stung. This is possibly due to its mucus that does not trigger the nematocysts. The shepherd fish seems to avoid the larger, stinging tentacles. It feeds on the smaller tentacles under the gas bladder. These fish benefit from the shelter of the stinging tentacles. It also helps the man o' war: the presence of these species may attract other fish to feed on.