First discovered in the late 19th century in the Mediterranean Sea, Turritopsis dohrnii is a small, transparent jellyfish, no larger than the nail on your pinky. Inside its translucent body is a bright red spot, which functions as its stomach and is equipped with up to 90 white tentacles. On the outside there is nothing to suggest that it is in any way different from the numerous other species of jellyfish around the world. Nevertheless, T. dohrnii has a unique ability, because of which scientists claim it has the potential to be biologically immortal.
Like all jellyfish, the life of T. dohrnii starts of as a small larva, called a planula. This tiny creature swims through the water freely, until it finds a suitable place on the ocean floor. There it grows into a colony of polyps – many genetically identical jellyfish, stacked on top of each other. These spawn into individual specimens, which after detaching start to grow, eventually mature, reproduce and finally die. So far nothing extraordinary. But if at any point throughout this journey the jellyfish encounters a crisis, like being physically damaged, is exposed to an extreme change in its environment or starts dying from starvation, it begins to transform all of its cells into a younger state. In other words, it goes through its life cycle backwards. It retracts its tentacles, shrinks in size and falls to the bottom of the ocean. There it turns again into a polyp, which spawns new genetically identical copies of itself. These newborn jellyfish can themselves repeat this cycle, meaning that there is no natural boundary to their lifespan.
This however doesn’t mean that it cannot die. If, for example, it’s eaten by a fish or infected by a disease, then it’s life will be over. And although they do not have to, most of them eventually succumb to old age. What is even more mysterious, nobody has ever witnessed its unique ability in nature. Because of the rate in which the process occurs and the extreme improbability of such a perfect timing, so far it has only been observed in controlled environments.