A jellyfish no wider than a fingernail cannot die of old age. When Turritopsis dohrnii gets old, starving, or injured, its adult cells transform and it shrinks back into a polyp - the larval stage it grew from years earlier. Then it grows up all over again, and can repeat the loop indefinitely. That makes it biologically immortal. Scientists only noticed in the 1990s, when aging adults kept reverting in their tanks instead of dying.
