The Portuguese Man o’ War (Physalia physalis) is a highly venomous open-ocean predator that resembles a jellyfish but is actually a siphonophore. Each man o’ war is actually a colony of several small individual organisms that each have a specialized job and are so closely intertwined that they cannot survive alone. In this manner, the larger colony consists of a float that keeps the colony at the sea surface, a series of long tentacles that are covered with stinging cells, a rudimentary digestive system, and a simple reproductive system.
This species is predatory. It uses its feeding tentacles to sting and paralyze small fishes, pelagic crustaceans, and other invertebrates. The feeding tentacles may be up to 50m long in some individuals! These tentacles deliver a powerful sting and are also used for defense against predators. Few species eat the Portuguese man o’ war, but some predators that specialize on stinging, for example, ocean sunfish, will eat them.
The Portuguese man o’ war is not valuable, commercially, and is common throughout the tropics. In some places, it is increasing in numbers, likely a result of changing open ocean food webs. This species’ sting can be very painful if encountered by people. When there are large numbers of individuals in an area, it is best to avoid swimming.