- Scientific name: Physalia physalis
- Size: Body up to 35 centimeters in length, although tentacles can be 20 metres or even longer.
- Distribution: Generally found in warm and tropical seas around the world, but is present off British and Irish coastline, especially in the south and west, in the warmer months of the year.
Despite being referred to as a jellyfish, the Portuguese man-o’-war is in fact a different species, as a true jellyfish is a single organism, and the Portuguese man-o’-war is actually a colony of separate polyps (organisms) that live together in the form of this species. However, the separate organisms that make up the this species are incapable of independent life and need to be together in the form of the Portuguese man-o’-war to survive. The name of this species comes from the supposed resemblance (when they are seen floating on the surface of the sea) to the man-of-war ships, used by the Royal Navy from the 1600s to the 1800s.
This species floats on the surface of the water by using a gas filled float. This is usually pink or purple in colour. Underneath this there is a single thick tentacle and a range of thinner tentacles. These tentacles contains very powerful stinging cells which can paralyse and kill fish which come into contact with them and will also cause immense pain in any humans that are unfortunate enough to touch them. At the very least contact with these tentacles will see the skin swell up with red lacerations and immense pain will result for at least a few hours. Fever and muscles spasms can also occur and breathing difficulties and even heart attacks can result. Although rare, humans have died as a result of being stung by a Portuguese man-o’-war, especially in people who are elderly or have underlying health conditions. Like the lion’s mane jellyfish, the stinging cells of the Portuguese man-o’-war remain active and capable of stinging for a long time after the creature has died, certainly for several days after death or even longer if the tentacles have remained damp or been repeatedly covered with water by the incoming tide. Despite the potent sting, many species of sea turtles feed on the Portuguese man-o’-war as they have skin which is too thick for the stinging cells to pierce. The ocean sunfish is also capable of consuming the Portuguese man-o’-war.
Many people are surprised to hear that the Portuguese man-o’-war can be found in British waters but it can indeed by present off the south west coasts of England and Ireland in the warm summer months, and appear to be increasing in numbers off the coastline of Wales in summer. In September 2017 sections of beaches across Cornwall were closed off to the public after an unusually large number of Portuguese man-o’-war jellyfish were spotted close to the shore. In the same month this species was also observed off beaches at Newquay and the Isles of Scilly. It is believed that warming sea temperatures and climate change may be causing the increasing numbers of this species in British waters.