Turritopsis dohrnii, commonly known as the immortal jellyfish, is a species of small, biologically immortal Medusozoa (jellyfish) found worldwide in temperate to tropic waters. It is one of the few known cases of an animal capable of completely reverting to a sexually immature, sedentary polyps colonial stage after having reached sexual maturity as a solitary individual. Like most other hydrozoans, T. dohrnii begin their lives as tiny, free-swimming larvae known as planulae. As a planula settles down, it gives rise to a colony of polyps that are attached to the sea floor. All the polyps and medusa arising from a single planula are 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 are known to prey on other jellyfish species at a rapid pace. If the T. dohrnii jellyfish is exposed to environmental stress, 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 practice individuals can still die. In nature, most Turritopsis dohrnii 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 maximum lifespan makes T. dohrnii an important target of basic biological aging and pharmaceutical research.
Taxonomy The species was formerly considered conspecific with T. nutricula before being reclassified as a separate species. It was named in 1883 in honour of Anton Dohrn, the founder of the Stazione Zoologica Anton Dohrn in Naples, Italy. Until a 2006 study, it was thought that Turritopsis rubra and Turritopsis nutricula were the same species as Turritopsis dohrnii. It is not known whether or not T. rubra medusae can also transform back into polyps.
Description The medusa of Turritopsis dohrnii is bell-shaped made of mesoglea, with a maximum diameter of about 4.5 millimetres (0.18 in) and is about as tall as it is wide. The mesoglea 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 are also present in the epidermis in the cap. They form a large ring-like structure above the radial canal 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.
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 not 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 Maria Miglietta. Turritopsis are able to withstand extreme conditions, such as lack of food and harsh environments which would normally be fatal due to their ability to regress into a cluster of cells, possibly being carried by water currents. It is in this way that this species has invaded many environments by being so ecologically efficient and stronger than competing species.
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. However, 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 jellyfish, 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 (68 °F) it is 25 to 30 days and at 22 °C (72 °F) it is 18 to 22 days). Medusae of T. dohrnii are able to survive between 14 and 25 °C (57 and 77 °F).
Biological immortality
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