There is a species of jellyfish that theoretically live forever; it is called the Turritopsis dohrnii. It lives in the Mediterranean Sea and around the waters of Japan.
In 1988, a marine biologist named Christian Sommer made a critical discovery about the Turritopsis dohrnii while studying hydrozoans, which at certain times of their life cycle resemble jellyfish. Sommer noted that the Turritopsis dohrnii has the ability to age in reverse once it reaches its oldest stage. The Turritopsis dohrnii grows younger and younger until it reaches the very earliest stage of its life cycle, at which point it starts again.