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 a metazoan 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. Cell transdifferentiation is when the jellyfish “alters the differentiated state of the cell and transforms it into a new cell”. In this process the medusa of the immortal 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.
Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal, although in nature, most Turritopsis, like other medusae, are likely to succumb to predation or disease in the plankton stage, without reverting to the polyp form.
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. (via Turritopsis nutricula - Wikipedia, the free encyclopedia)