Turritopsis nutricula is an immortal jellyfish. After reaching sexual maturity, this jellyfish is able to reverse its aging process and become a polyp again. The ability to reverse the life cycle is probably unique in the animal kingdom, and allows the jellyfish to bypass death, rendering the Turritopsis biologically immortal. Lab tests showed that 100% of specimens reverted to the polyp stage though apparently only as an emergency measure.
About as wide as a human pinky nail when fully grown, the immortal jellyfish was discovered in the Mediterranean Sea in 1883. But its unique ability was not discovered until the 1990s. Turritopsis typically reproduces the old-fashioned way, by the meeting of free-floating sperm and eggs. And most of the time they die the old-fashioned way too.
But when starvation, physical damage, or other crises arise, "instead of sure death, Turritopsis transforms all of its existing cells into a younger state. The jellyfish turns itself into a blob like cyst, which then develops into a polyp colony, essentially the first stage in jellyfish life. The jellyfish's cells are often completely transformed in the process. Muscle cells can become nerve cells or even sperm or eggs.
Through asexual reproduction, the resulting polyp colony can spawn hundreds of genetically identical jellyfish—near perfect copies of the original adult. This unique approach to hardship may be helping Turritopsis swarms spread throughout the world's oceans.