By now I'm sure everyone who's interested has heard about Diana Nyad's failed attempt at swimming from Cuba to Florida. The animals that stopped her with their paralyzing stings have been variously identified as Portuguese men-of-war (by Nyad herself in the video above) and box jellyfish. The man-of-war isn't technically a jellyfish, but a colony of hydroids. In The Book of Deadly Animals, I reported that at least three people have died from their stings. That's a tiny percentage, but pretty much everyone agrees the stings are painful. As for box jellyfish: many different species exist, some of them among the most toxic animals in the world, others not so dangerous to people.
All of these creatures--Portuguese men-of-war and other hydroids, and box jellyfish, plus the not-suspected-in-this-case true jellyfish and sea anemones--are members of a group called cnidarians. They all have stinging cells they use for self-defense and to help them kill prey. Nyad and her fellow humans are too large to serve as their prey, but these guys literally have no brain and aren't aware of their limitations. On the other hand, Nyad herself speaks of dreaming beyond one's expectations. It sounds inspiring when she says it, not so much when the cnidarians do it.
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