What animal will never die unless it's killed?

The animal said to never die unless it is killed is the jellyfish. More precisely, the “immortal jellyfish” is Turritopsis dohrnii, a tiny species capable of reversing its life cycle.

When adult Turritopsis dohrnii is injured, starving, or otherwise stressed, it can transform its mature medusa cells back into an earlier polyp stage. The polyp can then grow into a new adult, allowing the cycle to repeat rather than ending in ordinary biological aging.

This ability is called biological immortality, not invincibility. The jellyfish can still be eaten by predators, destroyed by disease, or killed by environmental damage. Scientists also do not claim that every jellyfish has this ability: it is associated especially with Turritopsis dohrnii, not the entire jellyfish group.

The species is therefore a better answer than “hydra” or “flatworm,” which also show unusual regeneration or aging patterns. “Never die unless killed” is a popular shorthand for its potentially unlimited life cycle.