A type of jellyfish – the turritopsis nutricula – has the potential to be immortal. The creature can revert to the polyp stage after becoming sexually mature. It is the only known case of a multi-cellular animal capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage.
It does this by cell transdifferentiation. In this process the medusa of the jellyfish is transformed into the polyps of a new polyp colony. First, the umbrella reverts itself and then the tentacles and mesoglea get resorbed. The reverted medusa then attaches itself to the substrate by the end that had been at the opposite end of the umbrella and starts giving rise to new polyps to form the new colony.
In theory, this process can go on infinitely, effectively rendering the jellyfish biologically immortal. In nature however, it is most likely to die from predation or disease in the plankton stage. No single specimen has been observed for any extended period, so it is not currently possible to estimate the age of an individual, and so even if this species has the potential for immortality, there is no laboratory evidence of many generations surviving from any individual.