The Portuguese man o' war is a highly venomous open ocean predator that superficially 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.
The Portuguese man o' war is a predatory species. It uses its feeding tentacles to sting and paralyze small fishes, pelagic crustaceans, and other invertebrates. The feeding tentacles may be up to 160 feet (50 m) long in some individuals! These tentacles deliver a powerful sting and are also used for defense against predation. Few species eat the Portuguese man o' war, but some predators that specialize on stinging, gelatinous invertebrates (e.g., loggerhead sea turtles and ocean sunfish) are known to feed on this and other siphonophores.
Each individual Portuguese man o' war is either a male or a female, and they reproduce sexually via a method known as broadcast spawning. Large groups of individuals come together, where females release their eggs and males release their sperm into the water column, all at the same time. This method increases the likelihood that eggs will be fertilized.
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.