This is Turritopsis nutricula, the immortal jellyfish.
Adult can actually revert back to the polp stage – they absorb their tentacles and reattach themselves the ground. As a polyp, it can produce entire new colonies. Lab tests found that 100% of individuals reverted to polyp stage, and could be prompted to do as a result of starvation, temperature change or a drop in salinity.
Biologically, Turritopsis nutricula is immortal.