Turritopsis nutricula, the immortal jellyfish, is a hydrozoan whose medusa, or jellyfish, form can revert to the polyp stage after becoming sexually mature. It is the only known case of an animal capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage. It does this through the cell development process of transdifferentiation, changing one type of cell into another.
It was discovered in the Mediterranean Sea in 1883, but its unique regeneration was not known until the mid-1990s. But Turritopsis can and do die. Their regeneration only occurs after sexual maturation, therefore they can succumb to predators or disease in the polyp stage.