Jellyfish: size, sting and the animal that never has to die

A jellyfish is a negotiation between water and almost-water. Between two thin layers of cells lies mesoglea — a jelly that is essentially structured sea. There is no brain, only a nerve net that lets the bell pulse and the animal respond to light, touch and gravity. Yet this minimalism is a triumph of engineering: the pulsing bell creates vortex rings that pull the animal forward and drag food toward its mouth, making jellyfish locomotion one of the most energy-efficient movements in nature. Engineers have copied it for underwater vehicles.

Stinging is equally refined. Each tentacle carries thousands of cnidocytes — cells containing a coiled, pressurised harpoon that fires in a millionth of a second when touched, among the fastest events in biology. The mauve stinger, Pelagia noctiluca, uses this arsenal on plankton and small fish; on humans the effect is a burning welt and a lesson in watching the water before swimming. Pelagia is also bioluminescent: disturbed at night, it flashes cold blue light — the "night-light" its scientific name records.

Why do jellyfish sometimes fill entire bays? A jellyfish bloom begins quietly on the seafloor, where the animal’s other life stage — the polyp, a small stalked creature few swimmers ever notice — waits, sometimes for years. When warmth, food and calm water align, each polyp buds off young medusae, and thousands arrive at once. Overfishing removes their competitors and predators; warming extends their season. The Mediterranean’s increasingly frequent mauve stinger invasions, which the Oceanographic Institute has studied for decades, are less an anomaly than a message from a changing sea.