What is a Portuguese Man o’war? Is a jellyfish or not?

Portuguese man o'war
Portuguese man o’war. Imagen de Olaf Gradin

It is very common to mistaken Portuguese Man o’war (Physalia physalis) with jellyfish when it is not a jellyfish, not even a living being… it is several beings. Portuguese man-of-wars are siphonophores, a colony of organisms that associate in order to survive. Each of these organisms, called zooids, is specialized in an activity necessary for the survival of the colony. All of these organisms are linked together and physiologically integrated and are unable to survive independently. 

What is a Portuguese Man o’war?

The Portuguese man o’war gets its common name from the upper polyp, a kind of gas-filled bladder, which can protrude up to 15 centimeters above the surface of the water and somewhat resembles a 16th century Portuguese warship. These organisms have the job of navigating. The long, slender tentacles, which can reach 50 meters in length, are covered with venom-laden nematocysts and are used to paralyze and kill fish and other small creatures on which they feed.

For humans, the sting of a Portuguese man o’war is very painful but rarely fatal and the pain disappears after a couple of hours. Be very careful whether you find one in the sea or dead on the shore, even if inert, its venom is active and can do a lot of damage. The fourth member of the colony is in charge of reproduction.

Portuguese man o’war can sometimes be found in large groups of up to 1,000 individuals, especially in warm waters. These colonies drift as they do not have their own means of propulsion and move either by currents or by wind.

To avoid threats on the surface, they are able to deflate the sail and dive briefly until the danger passes. Predators of these colonies include loggerhead turtles, leatherback sea turtles and sunfish. Both, thanks to the thickness of their skin, are immune to the caravel’s venomous tentacles. The glaucus atlanticus or blue dragon is an awesome nudibranch that also feeds on Portuguese man o’war, adopting their blue color and venom, which makes it easier for it to be ignored by other predators, sensing its toxicity.

Portuguese Caravels are most commonly found in the tropical and subtropical regions of the Pacific and Indian Oceans and the North Atlantic. Although they normally inhabit the open ocean, they can also be seen in the Mediterranean. 

One more fact about the Portuguese man o’war is that it is sometimes accompanied by a small fish, the Nomeus gronovii, known as Portuguese shepherdfish, which is partially immune to the venom of the stinging cells… but only of the smaller tentacles. It could die if it touches the larger tentacles that hold a larger amount of venom. That is why it lives and feeds on the smaller tentacles of its host’s swim bladder. It has a clownfish-like mucus that prevents it from being stung by anemones.


If you have suffered a sting from a Portuguese man’o war the first thing to do is to try to carefully remove the remains of the tentacles that may be left on the skin and immediately apply salt water, never fresh water. Then wash the area with hot water for a quarter of an hour, which will alleviate the pain of a sting. Vinegar should be applied if there are no remains of tentacles, but it is not advisable to apply vinegar if there are remains as it causes a discharge of the nematocyst.

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