The turritopsis nutricula species of jellyfish is capable of cycling from a mature adult stage to an immature polyp stage and back again, there may be no natural limit to its life span. It cant basically make its self a little baby jelly fish again after being a adult. They are the only known animals that can do this. They are immortal.
The key lies in a process called transdifferentiation, where one type of cell is transformed into another type of cell. Some animals can undergo limited transdifferetiation,like salamanders, which can grow back their tail. This Jellyfish, can regenerate its entire body over and over again.
Because they are able to bypass death, the number of individuals is spiking. These Jellyfish are now found in oceans around the world. It's kinda creepy. Of all things, our world is being taken over by immortal jellyfish. It is kind of cool though. They don't just die.