Portuguese man-of-war

Portuguese man-of-war, (Physalia physalis), species of jellylike marine animals of the order Siphonophora (class Hydrozoa, phylum Cnidaria) noted for its colonial body, floating habits, and powerful stings. It is often mistaken for a jellyfish. The Portuguese man-of-war’s prominent physical feature is its pneumatophore, a gas-filled bladder on its top that looks like the inflated sail of a ship and serves as both a float and a sail, allowing the animal to achieve maximum drift on the surface of the water.

The species is named for its resemblance to a class of warships that was developed in Portugal beginning in the 15th century with the caravel, a light agile sailing ship, called a man-o-war, that efficiently harnessed the wind. Physalia physalis is the only accepted species; however, some sources classify a regional form—the bluebottle, also called the Indo-Pacific man-of-war—that occurs in the Pacific and Indian oceans near Australia as a separate species called P. utriculus.

Natural history

The Portuguese man-of-war, although found in warm oceans and seas throughout the world, occurs most commonly in the Gulf Stream of the northern Atlantic Ocean and in the tropical and subtropical regions of the Indian and Pacific oceans; it is sometimes found floating in swarms of more than a thousand. With warming ocean temperatures and decreased oxygen in the water, its range is expanding, and reports of strandings and sightings in areas like Canada and Scotland, where the water has usually been too cool for the species, are increasing.

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The Portuguese man-of-war is a colonial organism made up of numerous specialized polyps, and four separate kinds of polyps constitute its structure: the uppermost polyp (pneumatophore), the tentacles (dactylozooids), the digestive polyp (gastrozooids), and the reproductive polyp (gonozooids). Each part performs its very specific bodily function. The pneumatophore—which may be 9 to 30 cm (3 to 12 inches) long and 12.7 cm (5 inches) wide and may project 15 cm (6 inches) above the water—is a translucent structure tinted pink, blue, or violet. The gases inside the pneumatophore include carbon monoxide, oxygen, nitrogen, and argon. Beneath the float are clusters of polyps, from which hang several opaque tentacles that can reach more than 30 metres (100 feet) in length. The bluebottle form, in contrast, possesses a single tentacle.

The species is part of the pleuston, a group of organisms that live at the ocean surface. The Portuguese man-of-war relies on ocean currents and wind for drift, and the colony cannot move on its own, whereas jellyfish are individual organisms capable of independent movement. Although the Portuguese man-of-war’s movement depends largely on the wind pushing its pneumatophore, this structure can be deflated quickly in order to submerge and escape predators or to keep from drying out in the sun.

The Portuguese man-of-war captures its prey by using its tentacles, or dactylozooids, which bear nematocysts, venom-filled stinging structures that paralyze small fishes, plankton, crustaceans, and other prey. Nematocysts come in two sizes: 11 and 24 nanometres (one nanometre is equal to one-billionth of a metre). After its victim is immobilized, the gastrozooids attach themselves to the victim’s body, spread over it, and digest it.

The Portuguese man-of-war in turn is eaten by other animals, including the loggerhead sea turtle (Caretta caretta), octopi, sea slugs, ocean sunfish (mola), and crabs. Some animals, like the common clown fish (Amphiprion ocellaris), have some immunity to stings and may feed among the tentacles. The man-of-war fish (Nomeus gronovii), which is about 8 cm (3.1 inches) long, lives among the tentacles of the Portuguese man-of-war, but it is not immune to the venom. It relies on being just fast enough to avoid being struck by the stinging cells; however, sometimes the Portuguese man-of-war captures it and eats it. The man-of-war fish feeds on the tentacles, which are constantly regenerated, and the gonads of the Portuguese man-of-war.

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The species uses broadcast spawning to reproduce. Each individual colony of polyps is either entirely male or entirely female. In the fall, in groups, females release eggs and males release sperm, and fertilization occurs near the water surface. The Portuguese man-of-war has an estimated life span of one year.

Interactions with humans

Incidents of Portuguese man-of-wars stinging people are common, and the stings are very painful (often resulting in red welts and blisters) and can have serious effects, including fever, shock, and interference with heart and lung activity. Deaths from Portuguese man-of-war stings are rare; deaths may result from allergic reactions to the venom or from cardiovascular incidents. The tentacles of dead Portuguese man-of-wars can still sting, making even dead animals dangerous to touch. Detached tentacles from deceased animals have been reported to still be capable of delivering stings.

Because Portuguese man-of-wars are subject to ocean currents, winds, and extremes of weather, large numbers of the animals often wash up on beaches, and their dispersal and territory may be wildly different in successive years. In 2010, for example, the Mediterranean Sea, Iberian Peninsula, and Canary Islands recorded 100,000 Portuguese man-of-war colonies; however, fewer than 60 colonies were sighted the year prior and the year after. Such overwhelming numbers can be problematic for tourism, beaches having to close to the public in order to avoid people coming into contact with these hazardous animals.