The jellyfish that knows how to grow young again

There is a tiny jellyfish, the size of a fingernail, capable of something no other complex animal can do: putting its own life into reverse.

It is called Turritopsis dohrnii. When it grows old, falls ill or suffers a serious injury, its cells transform and its whole body returns to the polyp stage, its juvenile form fixed to the sea floor. From there it grows again, like someone rewinding the tape and starting over.

In theory it could repeat that cycle without end, and that is why they call it the immortal jellyfish. In practice, almost all of them end up eaten or diseased. But its trick fascinates the scientists who study ageing: nature had already invented the reset button.