Most animals have a simple life plan. You're born, you grow up, you reproduce, you age, you die.

Turritopsis dohrnii read that plan and decided it was more of a suggestion.

Small, see-through, and stubborn

It's not much to look at. The adult — the bell-shaped stage we think of as a jellyfish, called a medusa — is about 4.5 millimetres across, smaller than your little fingernail. It's transparent, with a red stomach visible through its body like a cherry in jelly.

But when an adult Turritopsis dohrnii is starving, injured, stressed or simply old, it can do something almost no other animal can.

It sinks to the sea floor and shrinks. It absorbs its tentacles. It turns into a small blob of tissue, a sort of cyst. And then, over the following days, that blob grows into a polyp — the juvenile, plant-like stage that jellyfish start their lives as.

The polyp then buds off new, genetically identical baby jellyfish. And the cycle starts again.

The accidental discovery

The trick was first noticed in the late 1980s by Christian Sommer, a German marine-biology student studying small hydrozoans he had collected off the Italian Riviera. He kept them in dishes and watched what happened.

What happened made no sense. Some of his jellyfish seemed to vanish — and polyps appeared in their place.

It took several years and a team of Italian researchers, including Ferdinando Boero and Stefano Piraino, to work out what Sommer had seen. Their 1996 paper had a beautifully direct title: Reversing the Life Cycle.

How do you un-grow?

The key is a process called transdifferentiation. As animals develop, their cells specialise — becoming muscle, nerve, skin — and normally they stay that way. In Turritopsis, specialised cells can switch identity, turning into different kinds of cells as the body rebuilds itself into a polyp.

A 2022 study comparing its genome with that of a close relative that can't rejuvenate found that Turritopsis dohrnii has extra copies of genes involved in DNA repair and replication, and in protecting the tips of chromosomes. In other words, it seems unusually well equipped to keep its genetic instructions intact through repeated rebuilding.

Why it matters

Nobody is going to turn humans into polyps. But scientists who study ageing and regeneration are fascinated by an animal that can reset its own cells — and by what that tells us about how fixed "growing up" really is.