Immortal jellyfish, also called Turritopsis dohrnii, is a species of tiny, biologically immortal jellyfish located worldwide in tropical to temperate waters. It is one of the few identified cases of animals capable of returning completely to a sexually immature, colonial stage after having reached sexual maturity as a single individual.
The Turritopsis dohrnii Medusa is bell-shaped, with the highest diameter of about 0.18 inches (4.5 millimeters), and is about as wide as it is tall. The mesoglea in the bell walls is consistently thin, except for some unusual thickening at the apex.
Most jellyfish species have a comparatively rigid life span, varying by species from hours to some months (long-lived mature jellyfish spawn every day or night; the time is also somewhat limited and species-specific). The Medusa of Turritopsis dohrnii is the only form known to have acquired the ability to return to a polyp state by a particular transformation process that needs the carriage of certain cell kinds.
Experiments have shown that all stages of the medusae, from anew released to fully developed individuals, can convert back into polyps under the provisions of starvation, unexpected temperature change, artificial damage of the bell with forceps or scissors, or reduction of salinity. The transforming Medusa is described first by the deterioration of the bell, tentacles, and mesoglea. All immature Medusa then turned into a cyst-like stage and then transformed into stolons and polyps. However, about 20%-40% of mature Medusa went into the stolons and polyps frame without passing the cyst-like stage. Polyps were formed after two days since stolons had grown and fed on food. Polyps further multiply by growing new stolons, branches, and then polyps to create colonial hydroids. They would ultimately transform into polyps and stolons in the trial and start their lives once again, even without injury or environmental changes.
This capacity to reverse the biotic cycle (in response to unfavorable conditions) is unparalleled in the animal kingdom. It enables the jellyfish to bypass death, presenting Turritopsis dohrnii probably biologically immortal. The method has not been seen in their natural habitat because the process is relatively rapid and because field investigations at the right moment are unlikely. Regardless, most medusae are likely to fall victim to the everyday hazards of life as meroplankton, including succumbing to disease or being eaten by predators.