The jellyfish that's biologically immortal

There is an animal that, when injured, starving, or simply old, reverses its life cycle and grows young again. As far as anyone has been able to determine, it can repeat this trick indefinitely.

The animal is *Turritopsis dohrnii*, a jellyfish smaller than a fingernail, found in oceans from the Mediterranean to Japan. Most jellyfish hatch from a polyp on the seafloor, mature into the familiar swimming bell, reproduce, and die. *Turritopsis* can stop that arrow and reverse it. A stressed adult will sink to the bottom, reabsorb its tentacles, and re-form itself into a fresh polyp — a feat called *transdifferentiation*, in which fully specialised cells revert to an earlier developmental stage and then re-specialise into different tissues.

Human cells almost never do this. When they do, we call it cancer. *Turritopsis* does it on demand, cleanly, as a survival strategy.

It can still be eaten, crushed, or infected — biological immortality is not invulnerability. But left alone, the same individual jellyfish could, in principle, keep cycling between adult and polyp forever. The oldest one alive today might be a great deal older than the marine biologists studying it.