Turritopsis nutricula

Turritopsis nutricula , the potentially immortal jellyfish, is a hydrozoan whose medusa, or jellyfish, form can revert to the polyp stage after becoming sexually mature. It is the only known case of a metazoan capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage. It does this through the cell development process of transdifferentiation. 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, although in nature, most Turritopsis, like other medusae, are likely to succumb to predation or disease in the plankton stage, without reverting to the polyp form.. No single specimen has been observed for any extended period, so it is impossible to estimate the age of an individual, and so even if this species has the potential for immortality, there is no laboratory evidence of many generations surviving from any individual.

Biological immortality

Most jellyfish species have a relatively fixed life span, which varies by species from hours to many months (long-lived mature jellyfish spawn every day or night [the time is also rather fixed and species-specific]). The medusa of Turritopsis nutricula is the only form known to have developed the ability to return to a polyp state, by a specific transformation process that requires the presence of certain cell types (tissue from both the jellyfish bell surface and the circulatory canal system). 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. (read more)