Physalia physalis
Portuguese man-of-war

This species has been found in the Atlantic Ocean, the Pacific Ocean, the Indian Ocean, the Caribbean, and the Sargasso Sea. It floats on or near the surface of the water.

The Portuguese man-of-war floats on the surface of tropical, marine waters. Generally, these colonies live in warm tropical and subtropical water such as along the Florida Keys and Atlantic coast, the Gulf Stream, the Gulf of Mexico, the Indian Ocean, the Caribbean Sea, and other warm areas of the Atlantic and Pacific oceans. They are especially common in the warm waters of the Sargasso Sea.

The Portuguese man-of-war is a floating hydrozoan. It is actually a colony consisting of four types of polyps: a pneumatophore, or float; dactylozooids, or tentacles; gastrozooids, or feeding zooids; and gonozooids which produce gametes for reproduction. Nematocysts (stinging cells) are located in the tentacles. Their action is based on their individual osmotic and hydrostatic pressure. Sensory cells are numerous and are located in the epidermis of the tentacles and the region around the mouths. Generally, the sensory cells are receptors for touch and temperature.

The stinging cells, or nematocysts, are the characteristic food-getting mechanisms of jellyfish and their close relatives. P. physalis has two sizes of nematocysts, some small and others are large. These nematocysts retain their potency long after an individual has been washed up along the shore, as many hikers along beaches have discovered to their dismay and discomfort.

Locomotion is generally passive, driven by wind and current. The colony cannot swim, but floats by the aid of its pneumatophore, or float. The float is a long, gas-filled bladder, formed as an overgrown polyp in the shape of a closed bag. Some Men-of-War are "left-sided," while others are "right-sided." The "left-sided" individual drifts at an angle of 45 degrees to the right of the direction from which the wind is blowing, and the "right-sided" individual does the opposite. This distinction is crucial in the spreading of the animals more evenly over the warm oceans of the world.

The Portuguese Man-of-War traps its food in its tentacles. It feeds mainly on fish fry (young fish) and small adult fish, and it also consumes shrimp, other crustaceans, and other small animals in the plankton. Nearly 70 to 90% of the prey are fish.

The tentacles, or dactylozooids, are the Man-of-War's main mechanisms for catching its prey and are also used for defense. P. physalis sometimes traps and consumes larger fishes such as flying fish and mackerel, though fishes as large as these generally manage to escape from the tentacles. The food of the Man-of-War is digested in its bag-like stomachs (gastrozooids), which are located along the underside of the float. The gastrozooids digest the prey by secreting enzymes that break down proteins, carbohydrates and fats. Each Man-of-War has multiple gastrozooids complete with individual mouths. After the food has been digested, any undigestible remains are pushed out through the mouths. The nourishment from the digested food is absorbed into the body and eventually circulates to the different polyps in the colony.

Physalia hurts tourists and tourism in areas where it is common, due to stings (of neurotoxins) from its nematocysts. Much money is spent each year to treat swimmers who have been stung by the nematocysts of individuals that have washed up on beaches. The inflammatory response resulting from Physalia stings is due to the release of histamines from mast cells within the victim.

The Portuguese Man-of-War is eaten by some fish and crustaceans (e.g. the sand crab) that can be of commercial value.

Physalia pile up and are abundant in the center of the waters of the Sargasso Sea due to the movements of the currents around it and the rotation of the Earth.

The Portuguese Man-of-War is the sole member of the Siphonophora with a unisexual colony; it is distinguished by a contractile, horizontal float. Although Siphonophora are generally considered to be the most specialized hydrozoans, some researchers claim that it is in fact the most primitive order, with the medusa and the polyp not fully differentiated. Additional support for this view comes from the observation that the regenerative powers of the man-of-war are poor, in contrast to most other jellyfish.

An important aspect of the Man-of-War's behavior are the symbiotic relationships between the man-of-war and species from several genera of fish: the genus Nomeus contains several, clownfish (commonly called the Man of War fish), and the yellow-jack. These fishes live within the tentacles of a Physalia and are rarely seen elsewhere. Some of the symbiotic fish species, particularly the clownfish, produce a slimy mucus on their skin that prevents the man-of-war from firing its nematocysts. The Nomeus species may not have protective mucus. When first introduced to a Man-of-War, a Nomeus fish initially swims near the surface and around the Physalia in a large circular pattern in both clockwise and counterclockwise directions. In addition, it avoids the larger dactylozooids and remains near the surface, feeding on the tentacles that lie just under the float.