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Turritopsis dohrnii, the immortal jellyfish, is a species of small jellyfish which is found in the Mediterranean Sea and in the waters of 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 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 medusa of Turritopsis dohrnii is bell-shaped, with a maximum diameter of about 4.5 millimetres (0.18 in) 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, while adult specimens have 80-90 tentacles. The medusa (jellyfish) is free-living in the plankton.
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 range
Turritopsis is believed to have originated in the Caribbean but has spread all over the world, 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 as ships are discharging ballast water in ports. Since the species is immortal, the number of individuals could be rising fast. "We are looking at a worldwide silent invasion", said Smithsonian Tropical Marine Institute scientist Dr. Maria Pia Miglietta.
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 1 mm 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 (68 °F) it is 25 to 30 days and at 22 °C (72 °F) it is 18 to 22 days).
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. The transforming medusa is characterized first by deterioration of the bell and tentacles, with subsequent growth of a perisarc sheet 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 dohrnii 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.
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.
Keeping T. dohrnii in captivity is quite difficult. Currently only one scientist, Shin Kubota from Kyoto University, has managed to keep alive a group of these jellyfish for a prolonged period of time. The animals have to 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 10 times.
Kubota regularly appears in Japanese media to talk about his immortal jellyfish and has recorded several songs about them.
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