The Portuguese Man O’ War (Physalia physalis) is named after 18th-century sailing ships because, supposedly, they resemble a ship at full sail.
Out in the open ocean, they look more like floating pink party balloons, trailing long blue ribbons.
The balloon part is a float filled with carbon monoxide gas that acts like a sail. It stands up from the water’s surface and catches the breeze.
This is how Portuguese Man O’ Wars travel across the ocean, sometimes in aggregations of thousands. They don’t actively swim, but are entirely wind-powered.
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And they can be either right or left-handed, depending on which way their sail is aligned, relative to the wind direction.
Seen up close, they look a lot like jellyfish and have some things in common, including their painful stings.
Be careful if you see a deflated pale balloon on a beach with blue strings attached. It’s likely to be a dead Portuguese Man O’ War – they can lose the colours of their bodies after they die, but not the power of their stings.
The Portuguese Man O’ War is a siphonophore, a relative of true jellyfish, as well as sea anemones and corals.
There are around 175 species of siphonophores. Some live on the seabed, most swim through the deep sea and the Portuguese Man O’ War is the only species that floats at the surface.
The most unusual thing about siphonophores is the way they build their bodies. Like other animals, they begin life as a fertilised egg that grows into an embryo. After that, things get rather different.
A standard way for large animals to grow, including humans, is for the body to get bigger and parts of it to develop into specialised tissues and organs that perform certain functions, such as digestion and reproduction.
Siphonophores don’t do that. Instead, they clone themselves making smaller, genetically identical bodies.
Known as zooids, these stick together in specific patterns to form a colony that makes up the siphonophore.
Among many types of zooids, the main four are responsible for feeding, digestion, sex and stings.
A common misunderstanding is that zooids are individual animals, because some resemble relatives of theirs that live solitary lives, like the swimming medusae of jellyfish and the flower-like polyps of sea anemones.
But siphonophore zooids can’t survive on their own. They depend on the colony.
Portuguese Man O’ Wars are important parts of a little-known ecosystem, called the pleuston, that exists at the interface between sea and air.
As they drift, they catch fish and fish larvae in their tentacles (see below), which can stretch for 30m (almost 100ft), and paralyse their prey with venomous stings.
Other animals hunt for Portuguese Man O’ Wars. A type of sea slug, sometimes known as blue dragons, eats the tentacles, then moves the stings into their skin and uses them to defend themselves.
Blanket octopuses hunt for food and fend off attackers by brandishing pieces of Portuguese Man O’ War tentacles.
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