The Portuguese Man O’ War (Physalia physalis) is often mistaken for a jellyfish, with its bulbous body and stinging tentacles. But in fact it is a siphonophore, that is, a group of four highly specialised organisms called Zooids, working together as one colonial organism, incapable of survival on their own.
The pneumatophore acts as a gas-filled bladder that floats on the surface of the water. Without a propulsion system, the man o’ war relies on currents and tides to move around, although it can deflate the pneumatophore to temporarily fully submerge under water to escape attack.
The gonozooid is the component responsible for reproduction, and the gastrozooid takes care of feeding and digestion. The dactylozooid, the stinging tentacles, can be as long as 50 metres, and is anecdotally known to have one of the most painful stings out there, and in rare cases, death. The tentacles are primarily used to “fish”, containing venom-filled nematocysts which sting and kill small fish and shrimp.
Some small fish, such as the Shepard Fish (Nomeus gronovii), are actually immune to the venom and can live amongst the tentacles, feeding on the smaller dactylozooid beneath the gas bladder, in a symbiotic relationship where the small fish gain protection from predators, while attracting other small species for the man o’ war to feed from.
Photo credit: http://www.speciesblog.com/2012/09/03/portuguese-men-of-war/