Turritopsis nutricula, or the Immortal Jellyfish
Posted: Thu Aug 18, 2011 8:45 pm
Source~ http://en.wikipedia.org/wiki/Turritopsis_nutriculaWikipedia wrote:The medusa of Turritopsis nutricula 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 nutricula 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 buds 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.
Turritopsis is believed to have originated in the Caribbean but has spread all over the world, and has speciated into several populations that are difficult 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 spiking. "We are looking at a worldwide silent invasion" said Smithsonian Tropical Marine Institute scientist Dr. Maria Pia Miglietta.
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 rather fixed and species-specific]). The medusa of Turritopsis nutricula 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 (see hydroid) 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 nutricula 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.