Turritopsis Dohrnii

Turritopsis dohrnii is the only known animal to be immortal. Whenever it is injured or dying, it can turn its cells back into young cells and start anew.
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==History==
Formerly known as Turritopsis nutricula, the creature capable of such an amazing feat is a very specialized jellyfish. 
It belongs to the Phylum Cnidaria. Jellyfish can exist in polyp form, as medusa, or can alternate between both. T. dohrnii falls under the last category. Polyps are sessile creatures that stay attached to a substratum. They have a foot with which they attach to the substratum, and their mouth faces the water, surrounded by tentacles.
On the other hand, medusae are motile creatures that can swim about. They have a bell-shaped head with tentacles that help for swimming
As is the case with all jellyfish, they have neither a heart nor brain. They were first discovered in the Mediterranean Sea. Now they are spreading all over the world, particularly because these tiny creatures are hitchhiking on ships in their ballast water and traveling across the globe.
==Immortality reason==
While the life cycle of T. dohrnii moves in a set manner, these organisms have the unique ability to stop and reverse it. That’s correct. While in their medusa stage, the jellyfish can revert back to the polyp stage. They can do this not just while they are immature medusae, but even after they have attained sexual maturity!
They achieve this through a process called transdifferentiation. This is the ability of a mature cell to transform into another type of mature cell. This process requires the cells from the bell and the cells of the circulatory canal system. Therefore, an adult medusa can simply revert back to its polyp form after going through an intermediate cyst form, leaving it ready to start life all over again! This is biological immortality.
200px|center|Immortality through Transdifferentiation
However, it’s not all that easy. This conversion comes with certain conditions too; namely, medusae cannot completely control their reversal. Whenever the medusa is injured, the external conditions are not conducive, or if it is starving, only then can it undergo this transformation. Fortunately, there is no limit as to how many times it can undergo this change.
Even so, this doesn’t mean that they can never die. They can still be eaten or die of diseases, etc.
==References==
*Wikipedia.org
*National Geographic
*Marine Invasions Research
Category:Original species