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. 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.