After it reaches sexual maturity, it can go through a process of transdifferentiation and transform mature cells back to young cells (polyps). Here's one way to explain it:
Cell transdifferentiation is when the jellyfish "alters the differentiated state of the cell and transforms it into a new cell. In this process the medusa of the immortal jellyfish is transformed into the polyps of a new polyp colony. First, the umbrella reverts itself and then the tentacles and mesoglea get resorbed. The reverted medusa then attaches itself to the substrate by the end that had been at the opposite end of the umbrella and starts giving rise to new polyps to form the new colony. Theoretically, this process can go on infinitely, effectively rendering the jellyfish biologically immortal. (Wikipedia)
This little creature is about 4.5 mm in diameter (.18 inches). The red in the center is its large stomach. Young jellies have about 8 tentacles while adults have 80-90 tentacles. The picture shown below is actually a Turritopsis rubra from New Zealand which is closely related. They are very similar, but it's not known if T. rubra can transform back into polyps.
The jelly originated in the Carribbean but now it's found all over the world in temperate to tropical oceans. Because it's basically immortal (if it doesn't succumb to predation, etc), the numbers are spiking. They think it's spreading by ships discharging ballast water in ports.
A bit more about their immortality:
Careful laboratory experiments have revealed that all stages of the medusae, from newly released to fully mature individuals, can transform back into polyps. The transforming medusa is characterized first by deterioration of the bell and tentacles, with subsequent growth of a perisarc sheet (see hydroid) and stolons, and finally feeding polyps. Polyps further multiply by growing additional stolons, branches and then polyps, to form colonial hydroids. This ability to reverse the life cycle (in response to adverse conditions) is probably unique in the animal kingdom, and allows the jellyfish to bypass death, rendering Turritopsis nutricula potentially biologically immortal. Studies in the laboratory showed that 100% of specimens could revert to the polyp stage, but so far the process has not been observed in nature, in part because the process is quite rapid and field observations at the right moment in time are unlikely. In spite of this remarkable ability, most Turritopsis medusae are likely to fall victim to the general hazards of life as plankton, including being eaten by other animals, or succumbing to disease.
The hydra is also being studied because it also has the ability to regenerate. It can regenerate tissue when injured or severed and doesn't age.
Fascinating! The natural world is so wonderful and full of mystery. Maybe someday this beautiful little creature will help us all live longer, but if not, it's just at least exciting to know that it exists and thrives in our fantastic world.
Thanks to The Skeptic's Guide to the Universe for telling me about T. nutricula.