Immortal jellyfish biology is not a metaphor. Turritopsis dohrnii — a jellyfish smaller than a human fingernail — can reverse its own ageing cycle and return to its juvenile state after reaching full sexual maturity. Not once. Repeatedly. Indefinitely. In the entire catalogue of life on Earth, no other organism has been confirmed to do this. It is, by the strictest biological definition, potentially immortal. This is not science fiction. It is one of the most quietly astonishing discoveries in modern zoology — and the implications of what this tiny animal is doing at the cellular level are only beginning to be understood. What Turritopsis Dohrnii Actually Does Most jellyfish follow the standard arc of life: larva becomes polyp, polyp becomes medusa (the adult, free-swimming form), medusa reproduces and dies. Turritopsis dohrnii follows this path too — until something goes wrong. Under stress — starvation, physical damage, disease, even old…