Turritopsis dohrnii, the immortal jellyfish, is a species of small jellyfish which is found in the Mediterranean and in the waters off Japan. It is unique in that it exhibits a certain form of "immortality": it is the only known case of an animal capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage.
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 seafloor. Jellyfish, also known as medusae, then bud off these polyps and continue their life in a free-swimming form, eventually becoming sexually mature. All the polyps and jellyfish arising from a single planula are genetically identical clones. If a T. dohrnii jellyfish is exposed to environmental stress or physical assault, or is sick or old, it can revert back 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.
Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal, although, in nature, most Turritopsis are likely to succumb to predation or disease in the medusa stage, without reverting to the polyp form.
The "immortal jellyfish" was formerly classified as T. nutricula.