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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 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 individual. 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. 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. 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 "immortal jellyfish" was formerly classified as ''T. nutricula''.〔 == Description == The medusa of ''Turritopsis dohrnii'' is bell-shaped, with a maximum diameter of about 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. ''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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Turritopsis dohrnii」の詳細全文を読む スポンサード リンク
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