Turritopsis Nutricula is a species of Jellyfish that can age backwards.

Most jellyfish have a lifespan of between a few hours to a few months, while a few varieties can live for several years. The species Turritopsis Nutricula is an extraordinary exception that can potentially live forever. They grow old like every other living thing, but then have the unique ability to reverse this process and grow young again.

The Turritopsis Nutricula begins life much like most jellyfish. The male jelly will release its sperm into the water and it may unite with the egg of a female jelly. Eventually this will produce planulae (free swimming larva) that leave the body of the female jelly and float around until finally settling on the sea floor and attaching to something sturdy like a rock. It then forms into a stationary polyp that will eventually look like a plant. This polyp feeds on various things like plankton and over time a colony of polyps will form from the single polyp, all connected via feeding tubes. At a certain point, sometimes even years later, the colony of polyps will begin producing free swimming jellyfish.

Also like most jellyfish, the Turritopsis Nutricula, once formed into jellyfish, float away with the currents, gathering food as they encounter it.
The real miracle of this species is unveiled when food is scarce or they are injured or threatened by other environmental impacts. These events will trigger a unique mechanism within the Turritopsis Nutricula that causes it to begin to grow younger via transdifferentiation, where their cells begin to change themselves. Muscle cells can become sperm or eggs, or nerve cells can change into muscle cells, transforming each cell into a new type of cell.

They continue to grow young all the way to the point where they once again become a single polyp, starting the process all over again. They will then remain a polyp for a while and are even able to grow a new colony. Once again, at a certain point, free floating jellyfish form and are released from the polyp colony, each with the same genetic code as the original jellyfish that formed the polyp.
It is thought that this process can theoretically go on without end, but multiple factors can manage to interrupt the endless cycle: Their regeneration only occurs after sexual maturation; therefore they are most vulnerable in the polyp stage which is when most of these jellyfish fall prey to predators or die from disease.

However, in theory, they can live forever; the only known living thing known to be able to achieve this.

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