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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.

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.

Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal, However, it doesn't mean that they won't die. In nature, most Turritopsis are likely to succumb to predation or disease in the medusa stage, without reverting to the polyp form.

The capability of biological immortality with no lifespan makes T. dohrnii an important target of basic biological, aging and pharmaceutical researches.

The "immortal jellyfish" was formerly classified as T. nutricula.

Description
The medusa of Turritopsis dohrnii is bell-shaped, with a maximum diameter of about 4.5 mm and is about as tall as it is wide. The jelly in the walls of the bell is uniformly thin, except for some thickening at the apex. The relatively large stomach is bright red and has a cruciform shape in cross section. Young specimens 1 mm in diameter have only eight tentacles evenly spaced out along the edge, whereas adult specimens have 80–90 tentacles. The medusa (jellyfish) is free-living in the plankton. Dense nerve net cells also present in the epidermis in the cap. They form a large ring-like structure above the radial cannal which is commonly presented in cnidarians.

Turritopsis dohrnii also has a bottom-living polyp form, or hydroid, which consists of stolons that run along the substrate, and upright branches with feeding polyps that can produce medusa buds. These polyps develop over a few days into tiny 1 mm medusae, which are liberated and swim free from the parent hydroid colony.

Images of both the medusa and polyp of the closely related species Turritopsis rubra from New Zealand can be found online. Until a recent genetic study, it was thought that Turritopsis rubra and Turritopsis nutricula were the same. It is not known whether or not T. rubra medusae can also transform back into polyps.

Distribution and invasion
Turritopsis is believed to have originated in the Pacific but has spread all over the world through trans-Arctic migrations, and has speciated into several populations that are easy to distinguish morphologically, but whose species distinctions have recently been verified by a study and comparison of mitochondrial ribosomal gene sequences. Turritopsis are found in temperate to tropical regions in all of the world's oceans. Turritopsis is believed to be spreading across the world through ballast water discharge. Unlike other species invasions which caused serious economic and ecological consequences, T. dohrnii’s invasion around the world was unnoticed due to their tiny size and innocuity. "We are looking at a worldwide silent invasion", said Smithsonian Tropical Marine Institute scientist Dr. Maria Miglietta.

Life cycle
The eggs develop in gonads of female medusae, which are located in the walls of the manubrium (stomach). Mature eggs are presumably spawned and fertilized in the sea by sperm produced and released by male medusae, as is the case for most hydromedusae, although the related species Turritopsis rubra seems to retain fertilized eggs until the planula stage. Fertilized eggs develop into planula larvae, which settle onto the sea-floor (or even the rich marine communities that live on floating docks), and develop into polyp colonies (hydroids). The hydroids bud new jellyfishes, which are released at about one millimetre in size and then grow and feed in the plankton, becoming sexually mature after a few weeks (the exact duration depends on the ocean temperature; at 20 C it is 25 to 30 days and at 22 C it is 18 to 22 days). Medusae of T.Turritopsis are able to survive between 14°C  and 25°C.

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 fairly fixed and species-specific). The medusa of Turritopsis dohrnii 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 under the conditions of starvation, sudden temperature change, reduction of salinity and artifical damage of the bell with forceps or scissor. The transforming medusa is characterized first by deterioration of the bell, mesoglea and tentacles. All immature medusa (with 12 tentacles at most) then turned into a cyst-like stage and then transformed into stolons and polyps. However, about 20%-40% of mature medusa went into stolons and polyps stage without passing cyst-like stage. Polyps were formed after 2 days since stolons had developed and fed on food. Polyps further multiply by growing additional stolons, branches and then polyps, to form colonial hydroids. In the experiment, they would eventually transform into stolons and polyps and begin their lives once again even without suffering from environment changes or injury. Diagram of the transformation procedure can be found at the futher reading.

This ability to reverse the biotic cycle (in response to adverse conditions) is unique in the animal kingdom, and allows the jellyfish to bypass death, rendering Turritopsis dohrnii potentially biologically immortal. The process has not been observed in their natural habitat, in part because the process is quite rapid, and because field observations at the right moment 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 predators or succumbing to disease.

The Turritopsis dohrnii's cell development method of transdifferentiation has inspired scientists to find a way to make stem cells using this process for renewing damaged or dead tissue in humans.

Genomic analysis
Scientists have processed a series of genomic analyses on T. dohrnii, such as sequence analysis on mRNA or mitochondria DNA, and tried to unmask the secret of its immortality. One mRNA analysis research on each stage of T. dohrnii uncovers a stage specific gene in medusae stage of T. dohrnii is expressed 10 times more than in other stages. This gene is relative to a Wnt signal that can induce regeneration process when getting injury.

By using NCBI BLASTn to identify nucleotide sequence homologs and BLASTp to identify protein homologs, homologous sequence similarity search offered some related species. Nemopsis bachei is found to be closest relative of T. dohrnii. Also, the closest taxa based on the data are found. However, none of these species were reported to become immortal.

However, there are still many researches needed to be done for unmasking the secret of immortal jellyfish.

Culturing
Keeping T. dohrnii in captivity is quite difficult. Currently only one scientist, Shin Kubota from Kyoto University, has managed to sustain a group of these jellyfish for a prolonged period of time. The plankton must be inspected daily to ensure that they have properly digested the Artemia cysts they are being fed. Kubota reported that during a two-year period, his colony rebirthed itself 11 times. Kubota regularly appears on Japanese television to talk about his immortal jellyfish and has recorded several songs about them.

In popular culture

 * The Blacklist season 2 episode 17 "The Longevity Initiative" – a billionaire tech mogul whose fascination with immortality led him to develop and fund a research team for just that purpose. Leading the research is a scientist who has been injecting the cells of T. dohrnii into humans in order to hopefully force their cells to regenerate.
 * Zoo season 2 episode 6 "Sex, Lies and Jellyfish" – The team searches for immortal jellyfish to find a cure for animal mutations.
 * Wild Kratts Videos "Meeting the Immortal Jellyfish" – During the rescue of the Bowhead whale from Zach, Martin and Chris are thrown from the Tortuga, ending up in the Sea of Japan with a group of jellyfish. But these jellyfish are different.  They're called the Immortal jellyfish and can live forever.  With the help of Aviva,  Martin and Chris activate with Immortal jellyfish powers and soon discover the highs and lows of living forever.
 * Octonauts season 3 episode 17 "Octonauts and the Immortal Jellyfish" – The Octonauts encounter an immortal jellyfish which reverts back into a "baby jellyfish" and then matures into an adult again.
 * The design and abilities of the Vocaloid LUMi (ルミ) draw inspiration from T. dohrnii.
 * Progressive post metal band, The Ocean wrote the song, 'Turrtopsis Dohrnii', inspired by the jellyfish.