Most animals get one life, lived in one direction: born, grow up, grow old, die. One tiny jellyfish never got that memo.
Meet the immortal jellyfish. Turritopsis dohrnii is barely 4.5 millimetres across — smaller than your pinky nail — with a transparent bell, a bright red stomach, and up to 90 white tentacles. It was first described in the Mediterranean in the 1880s, and it now turns up in temperate and tropical seas around the world.
The reset button. Like other jellyfish, it starts as a free-swimming larva, settles on the seafloor as a colony of polyps, then buds off into the adult medusa form we recognise as a jellyfish. That is where the story should end. But when T. dohrnii is starved, injured, or otherwise stressed, it does something almost no other animal can: it reverses course.
The adult shrinks in on itself, reabsorbs its tentacles, and settles on the seafloor as a blob-like cyst. Over the next day or so, through a process called transdifferentiation, its specialised cells transform into new cell types and rebuild it as a polyp — its own childhood stage. That polyp then buds off new medusae, genetically identical to the original adult. In theory, the cycle can repeat indefinitely.
The fine print. "Immortal" is a nickname, not a guarantee. In the wild, most of these jellyfish are eaten or fall ill before they ever get to reset. No single individual has been watched living forever. Still, scientists study T. dohrnii closely, because an animal that can reprogram its own cells back to youth might one day teach us something about ageing — and about repairing damaged tissue.
Death, it turns out, is not always a one-way street. Sometimes it is just a detour back to the beginning.