Alaric Bond was kind enough to pass along an article from the Eastbourne Herald titled, Killer ‘jellyfish’ which can grow to 160ft long are washing up on Britain’s beaches. While more common in tropical waters, Portuguese man o’ wars have been drifting ashore near tourist areas in Cornwall, Sennen Beach, and Porteras Cove. 

The man o’ war have no independent means of movement and often move with the winds or sea currents. They are known to wash up on British shores between September and December.

Fortunately, while the article itself, about Portuguese man o’ war, is quite good, the headline writer is arguably guilty of some level of hyperbole. The good news is that while the man o’ war is a highly effective killer of small fish, they are rarely deadly to humans.

And as to growing to “160 feet,” the man o’ war’s iridescent gas-filled bladder, topped with a purple frill, is typically only about 6″ long and 5″ high. Beneath it, however, it trails long strands of venomous stinging tentacles and polyps that grow to an average of 30 feet and may extend by as much as 160 feet. What makes these tentacles extremely nasty is that they can still sting weeks after the man o’ war has washed ashore and died.

Human deaths from man o’ war are not unknown but are extremely rare. Nasty non-lethal stings are, however, not uncommon. On February 11, 2018, for example, 204 people in Hollywood, Florida were treated for stings, which can lead to red welts on the skin, muscle cramps, elevated heart rates, and vomiting. 

I have been stung twice by a man o’ war and lived to tell the tale. The first time was when I was around three or four years old. I was with my mother, playing in the surf at Miami Beach. The breaking waves carried shells and sand that hit our legs as we jumped in the breakers. My mother held my hands to make sure that I didn’t slip underwater. At one point, I screamed, “Mommy, a fish bit me.” She told me that it was just the shells tossed up by the surf. I did my best to stop crying and kept jumping in the waves.

When we got out of the water, my mother gasped. My legs and lower abdomen were marked with nasty purple stripes where a man o’ war’s tentacles had dragged across my body. My mother was understandably shocked, leaving me somewhat surprised. I said, “I told you that a fish bit me.”

The Deadly Portuguese Man O’ War | Blue Planet II | BBC Earth