<Tr> <Td_colspan="2"> Turritopsis dohrnii (Weismann, 1883) </Td> </Tr> <P> Turritopsis dohrnii, the immortal jellyfish, is a species of small, biologically immortal jellyfish found in the Mediterranean Sea and in the waters of Japan . It is one of the few known cases of animals capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary individual . Others include the jellyfish Laodicea undulata and Aurelia sp. 1 . </P> <P> Like most other hydrozoans, T. dohrnii begin their life as free - swimming tiny larvae known as planula . As a planula settles down, it gives rise to a colony of polyps that are attached to the sea - floor . All the polyps and jellyfish arising from a single planula are genetically identical clones . The polyps form into an extensively branched form, which is not commonly seen in most jellyfish . Jellyfish, also known as medusae, then bud off these polyps and continue their life in a free - swimming form, eventually becoming sexually mature . When sexually mature they have been known to prey on other jellyfish species at a rapid pace . If a T. dohrnii jellyfish is exposed to environmental stress or physical assault, or is sick or old, it can revert to the polyp stage, forming a new polyp colony . It does this through the cell development process of transdifferentiation, which alters the differentiated state of the cells and transforms them into new types of cells . </P> <P> Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal, although in practice individuals can still die . In nature, most Turritopsis are likely to succumb to predation or disease in the medusa stage, without reverting to the polyp form . </P>

Turritopsis nutricula immortal jellyfish is the only species known to live forever