What is a Portuguese man o' war, and why it is not a jellyfish?

⏱ Duration: 6:55 📅 Published on 2025-08-10 Watch on YouTube ↗

Translucent blue and violet, a diaphanous sail floating at the surface, long tentacular filaments trailing down into the depths… The Portuguese man o’ war, also known as the “bluebottle” or the “sea bladder“, is one of the most recognisable and strangest creatures in the ocean. Yet despite its appearance and its formidable stings, it is not a jellyfish. It is not even an animal in the classic sense of the word. It is a colony, a superorganism, and the way it works defies our usual categories.

An assembly of specialised polyps

The Portuguese man o’ war (Physalia physalis) belongs to the siphonophores, a remarkable order within the cnidarian phylum (the same phylum as jellyfish, anemones and corals). What looks like a single animal is in fact a colony of highly specialised polyps, genetically identical but with radically different functions, living attached to one another and unable to live apart.

This arrangement is unique in biology: the term “superorganism” is used because no polyp can exist on its own, and the whole functions as a single entity. Four main types of polyp make up the colony: the pneumatophores (the float), the dactylozooids (the capture tentacles), the gastrozooids (digestion) and the gonozooids (reproduction).

The sail and the sailing animal

The part visible at the surface, the bladder, is in fact the pneumatophore. It is a sac filled with a gas mixture that contains, among other things, carbon monoxide: a gas toxic to most animals, but used here as the main component of the float. This bladder can measure 15 to 30 centimetres and rises out of the water like a small translucent blue and violet sail.

The Portuguese man o’ war has no fins. It is carried entirely by the wind pushing on its float. It is one of the very few genuinely sailing animals: it crosses the oceans on the strength of atmospheric currents alone. More surprising still, it can trim its sail: by muscular contractions of the upper crest of the pneumatophore it alters how the wind catches it, and it can even submerge completely to escape surface turbulence for a time.

Right-handed and left-handed: an evolutionary trick

A fascinating discovery: there are right-handed and left-handed Portuguese men o’ war. Depending on how their sail is set relative to the float, some drift on average 45° to the right of the wind and others 45° to the left. These are not distinct genetic varieties, simply a random asymmetry that arises during development.

Why this feature? It has a remarkable ecological function: it disperses the colonies in different directions under one and the same wind. Imagine a group of Portuguese men o’ war driven towards a coast by the wind: if all of them went the same way, all of them would strand in the same place. With the right-handed/left-handed split, some go to the right and others to the left, which spreads the population out and avoids mass strandings. An elegant collective survival strategy, born of a simple anatomical asymmetry.

Deadly stinging filaments

Beneath the bladder hang the tentacles (dactylozooids), which can reach 10 to 30 metres in length in the largest colonies. These filaments are armed with cnidocysts: stinging cells that inject an extremely powerful neurotoxic venom. For fish, on which the Portuguese man o’ war feeds, contact is fatal. For humans it is an extremely painful experience, and in some cases (allergy, extensive contact) it can cause serious reactions.

The venom remains active several hours after the animal’s death: a Portuguese man o’ war stranded on a beach is still dangerous to touch. This is why bathers are regularly warned on Mediterranean and Atlantic beaches where these organisms may drift ashore. In the event of a sting, rinsing with seawater (never with fresh water, which makes the cnidocysts discharge more) and vinegar are the first recommended steps.

The float and its fish

Curiously, some animals have developed ways of exploiting the Portuguese man o’ war without suffering its effects. The man-of-war fish (Nomeus gronovii) lives among its tentacles without being stung, apparently thanks to a protective layer of mucus. It finds shelter there from its predators and feeds on scraps from the prey captured by its partner.

More surprising still, some sea turtles, the leatherback turtle in particular, actively feed on Portuguese men o’ war without suffering from their venom. Their thick skin and their resistant stomach protect them. This dietary specialisation partly explains why leatherback turtles are so vulnerable to floating plastic bags, which they mistake for their usual prey, a painful example of the consequences of marine pollution.

The Portuguese man o’ war is a reminder that the ocean holds forms of life that defy our categories: neither truly an individual animal nor a conventional colony, but a superorganism whose translucent beauty conceals a formidable efficiency.