jellyfish

English jellyfish appeared in the 1790s as a straightforward compound: the animal is soft and gelatinous (jelly-like) and lives in the sea (fish, in the broadest sense). Earlier English names included 'sea nettle' and 'sea blubber.' The word jelly itself comes from Old French gelee, from Latin gelare ('to freeze') — a frozen liquid that wobbles. The animal wobbles. The name fits the texture, if not the taxonomy.

Jellyfish are among the oldest multicellular animals on earth. Fossil evidence from the Ediacaran period (~600 million years ago) shows jellyfish-like organisms. They predate fish, insects, trees, and dinosaurs. They have no brain, no heart, no blood, and no bones. They are roughly 95% water. And yet they are one of the most successful animal body plans in the history of life.

The box jellyfish (Chironex fleckeri) of northern Australia is among the most venomous animals on earth. Its tentacles carry enough toxin to kill 60 adult humans. At least 64 deaths have been attributed to it in Australia since 1884. The turritopsis dohrnii, known as the 'immortal jellyfish,' can revert to its juvenile polyp stage after reaching maturity — effectively aging backward. One species kills. Another cheats death.

Jellyfish populations are increasing globally as ocean temperatures rise and overfishing removes their predators and competitors. Jellyfish blooms have shut down power plants (by clogging cooling water intakes), closed beaches, and collapsed fisheries. The animal with no brain is thriving in the conditions created by the animal with the biggest brain. The brainless blob is winning.