Most jellyfish die after they grow up and lay eggs. But one tiny jellyfish, only a few millimeters long, can do something amazing. When it is weak or hurt, it changes back into a polyp, its baby form.

Then it grows into an adult again. It may do this many times. But it is not really immortal, or unable to die. If a fish eats it, it dies.

Most jellyfish live in two forms. First there is a polyp, which sticks to rocks like a small sea anemone. Then there is a swimming adult. Usually, the adult dies after producing eggs. In 1996, European researchers reported that a tiny jellyfish called Turritopsis could run this cycle backward.

When stressed, the adult shrank into a ball, attached itself to the sea floor and turned back into a polyp. Its cells seemed to change from one type into another. The polyp could then produce new jellyfish, so in theory the cycle could repeat again and again. That is why people call it the "immortal jellyfish."

But it does not really live forever. In the wild, most are probably eaten or die of disease before they can become young again.

A jellyfish only a few millimeters across has earned the nickname "immortal jellyfish." Turritopsis, like many related animals, normally alternates between two forms. A polyp stays attached to the sea floor, and a free-swimming medusa, which typically dies after reproducing, buds off from it.

In 1996, Stefano Piraino and colleagues reported something unexpected. When stressed, the medusa could shrink its bell and tentacles into a ball, settle, and develop back into a polyp, which could later produce new medusae. The team proposed that transdifferentiation, the conversion of specialized cells into different cell types, made this reversal possible.

Not everyone accepts that explanation. Undifferentiated cells might be doing the work instead, and the question has not been settled. In 2022, a Spanish team compared the genome of Turritopsis dohrnii with that of a close relative that cannot rejuvenate.

Genes involved in DNA repair and in protecting chromosome ends were duplicated or altered, and gene activity shifted during rejuvenation. These are correlations, though, and they do not show that any particular gene is necessary. The nickname also overstates things. Predators and disease kill these jellyfish, and most individuals in the wild probably die before they ever reverse their life cycle.