This jellyfish can rewind its own life cycle and start over

Turritopsis dohrnii is a tiny jellyfish species that, in response to stress, injury, or aging, can undergo a process called transdifferentiation — its mature cells revert to an earlier developmental stage.

In practice, this means an adult jellyfish can "reset" itself back into a polyp — the stationary, juvenile form that normally develops into an adult jellyfish only once in a lifetime. In this species, the cycle can repeat over and over.

This doesn't make it literally indestructible — it can still be eaten by a predator or die of disease — but in theory, under the right conditions, it can avoid dying of old age indefinitely by rewinding its biological clock.

The bigger picture: this is the only known case of an animal biologically confirmed to reverse the aging process at the level of an entire organism, which makes it a major focus of research into cell regeneration.