The immortal jellyfish can reverse its own aging by returning to childhood. When stressed, sick, or old, Turritopsis dohrnii transforms itself back into a juvenile polyp colony through a cellular process called transdifferentiation, then grows up all over again. Theoretically, this cycle could repeat indefinitely, making it the only known animal capable of true biological immortality.

The jellyfish begins life as a free-swimming larva that settles and forms a branched colony of polyps on the sea floor. From these polyps, it releases jellyfish that grow into sexually mature adults. If conditions become harsh or the adult jellyfish ages, something remarkable happens: its cells reorganize and revert to the polyp stage, essentially hitting a biological reset button. The entire process can then repeat.

While the immortal jellyfish's superpower is real, it is not invincible in nature. Most individuals still die from predation or disease before they ever have the chance to revert. But the few that do survive long enough to trigger their rejuvenation process could theoretically live forever. Scientists study these jellyfish intensely because understanding how they reverse cellular aging could unlock secrets about aging in other animals, including humans.