Is a Portuguese Man o' War a Jellyfish? The Colony Explained

What makes something a true jellyfish?

In biology, “true jellyfish” usually means the class Scyphozoa. Members include the moon jellyfish, the lion’s mane and the barrel jellyfish. Each adult is a single individual called a medusa: a bell of stiff jelly with a mouth underneath, oral arms and a fringe of tentacles. It swims by squeezing its bell and pushing water out behind it.

Most true jellyfish also have a hidden second stage. Their larvae settle on the seabed and grow into small, fixed polyps, which later bud off tiny young jellyfish. Box jellyfish (class Cubozoa) are close relatives built on the same medusa plan. None of these animals are fish, as explained in Are jellyfish fish?

What is a siphonophore?

Siphonophores are an order of hydrozoans, the Siphonophorae, with more than 170 known species. Most live in the open ocean, many of them in the deep sea, and most are transparent, fragile and rarely seen. They swim with small jelly bells and trail long chains of zooids behind them.

What unites them is their colonial body. A siphonophore starts as one fertilized egg, which develops into a first zooid. That zooid then buds off many more, and each new zooid takes on one job. All the zooids stay physically connected and share food through a common stem, so the colony works as a single organism. The Portuguese man o’ war is the best known siphonophore because it is one of the few that lives at the surface, where people meet it.

What are the parts of a man o’ war colony?

Scientists have described about seven types of zooids in the Portuguese man o’ war, but they fall into four main working groups.

The float is filled mostly with air plus some carbon monoxide. The tentacles carry thousands of stinging cells called nematocysts. When a fish blunders into them, the tentacle shortens and drags it up to the gastrozooids, which spread over the prey and dissolve it. The nutrients then pass through the colony to the other zooids, including those that cannot feed themselves.

Is a man o’ war one animal or many?

The honest answer is both. From the point of view of development, a man o’ war is many individuals, because each zooid is built like a separate polyp or medusa. In other cnidarians those body plans form whole animals: a sea anemone is a polyp, and a jellyfish is a medusa. In the man o’ war, some reproductive zooids are reduced medusae that never swim away.

From the point of view of ecology, it is one animal. All zooids are clones that grow from the same egg, none can survive alone, and they act together to float, hunt, eat and breed. Biologists often compare the zooids to the organs of a single body and call the whole colony a superorganism. Like jellyfish, it does this without a brain, using simple nerve networks (see Do jellyfish have brains?).

How a man o’ war differs from a jellyfish

The two still have a lot in common. Both are cnidarians, both catch prey with nematocysts and both are made largely of water. Stinging cells keep working after death in both groups, so neither should be handled on a beach.

Other jellyfish look-alikes at the surface

Several other floating hydrozoans are mistaken for jellyfish or for small man o’ war. The by-the-wind sailor (Velella velella) is a flat blue disk a few centimeters across with a stiff, upright sail, and it sometimes strands in huge numbers on the coasts of western Europe and the Pacific coast of North America. It is not a siphonophore, and biologists still debate whether it is best viewed as a colony or a single giant polyp. The blue button (Porpita porpita) is a close relative of the by-the-wind sailor found in warmer seas.

Some deep-sea siphonophores grow far longer than any man o’ war. The giant siphonophore Praya dubia can stretch to tens of meters, and in 2020 scientists filmed a spiraling Apolemia colony in a deep canyon off Western Australia whose outer ring alone was estimated at about 47 m (154 ft).

Why it is still called a jellyfish

Because it floats, stings and looks like a blue bag of jelly, the man o’ war is often grouped with jellyfish by lifeguards, beach warning signs and everyday speech, and Australians often say “bluebottle jellyfish”. For safety purposes, that makes sense. For biology, it is more accurate to call it a siphonophore or simply a colonial hydrozoan.

Learn more about its size, diet, sting and range in our Portuguese man o’ war profile, or find out why man o’ war wash up on beaches.