Most jellyfish have two major stages in their life cycle: the immature stage, called polypoid, a small stalk with tentacles to absorb nutrients; and the mature stage, called medusa, when polyps only 1mm long can produce asexually. The lifespan of a jellyfish can range anywhere from hours, in the smallest species, to years in the larger.
This rejuvenation requires tissue from the ‘bell’ surface and circulatory system. This switching of cell roles is common in animals, and even humans, but it is only used when regenerating parts of organs.
The “immortal jellyfish” originates from the Caribbean, but is now found in tropical oceans around the globe, spreading through ship ballast water. In their adult form the Turritopsis nutricula range from about 4-5mm in diameter and polypoids are 1mm, they begin with 8 tentacles but can have up to 90.
Although the “immortal jellyfish” can be found around the world and are biologically immortal, jellyfish have many predators (sea turtles, ocean sunfish, tuna, sharks, swordfish, to name some), and are susceptible to diseases at their medusa stage. None of this is to discredit how amazing this discovery and jellyfish are; they are the only animal known to revert to an immature stage.