There is a species of jellyfish that is immortal. - Apparently.
The Turritopsis dohrnii, also known as the "immortal jellyfish," is a type of jellyfish that can transform its body into a younger state through a process called transdifferentiation. This means that it can essentially revert back into its polyp stage, which is the juvenile form of a jellyfish, and then grow back into an adult again. This process can be repeated indefinitely, making the Turritopsis dohrnii theoretically immortal. This phenomenon was first discovered in the 1990s.