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= Turritopsis nutricula = Turritopsis nutricula is a small hydrozoan endemic to the Carribean and western Atlantic Ocean. It is one of the only animals with the ability to return to its' juvenile life stage, giving it theoretical immortality. Similar organisms include the jellyfish T. dohrnii and T. rubra; both of which were once classified as T. nutricula

History
T. nutricula was first described in 1857 by John McCrady. The genus Turritopsis (family Oceaniidae), which now includes several distinct species, had previously been described as a synonymous Turritopsis nutricula. However, based on reproductive and morphological characteristics these species are clearly found to be different form T. nutricula. The misclassification of other Hydrozoa as T. nutricula was mostly due to it being the first Turritopsis species whose life cycle was fully described. Without knowledge of the full life cycles, many other hydroid colonies around the world were classified as T. nutricula. There are now a dozen discovered species in the genus Turritopsis, including Turritopsis dohrnii or "The Immortal Jellyfish" which posses a life cycle similar to that of T. nutricula allowing it to prolong its' own life.

Description and Distribution
T. nutricula possesses a deep bell shape, with a notched lateral outline approaching the peak of its dome. The bell is nearly transparent and the large muscular cells can be seen as light reflects off their edges. The bell shape is about 5 mm across and generally has the same width and height. The bell has a complex nerve ring structure with cell bodies of neurons. A large bright red stomach can be seen with a cross-shaped cross section. Younger specimens have only 8 tentacles while adults are observed to have greater than 80 tentacles. Turritopsis nutricula was first found in the Caribbean with populations spanning from temperate to tropical regions and are generally found in the western Atlantic. Species density has been observed to increase greatly following major storms in the Mid Atlantic. T. nutricula is not likely to be found on European coasts or Mediterranean and is often misidentified as T. dohrnii in these regions.

Life Cycle
The hydrozoan life cycle is characterized by two distinct stages in their life; a polyp stage and a medusa stage. The polyp stage is benthic, with the cells forming colonies, while the medusa stage is a singular, planktonic organism. Generally In hyrdrozoa the medusa develops from the asexual budding of the polyp and the polyp results from sexual reproduction of medusae. In T. nutricula, planktonic medusa have the capability to bud polyps or medusae which also have the ability to spawn new medusae. The lifespan of medusae would generally set a limit on the distribution of the hydra but the capability of budding and fission of medusae give the animal the ability to travel vast distances.

Turritopsis nutricula in any point of the medusa stage has the ability to transform back into its polyp stage. T. nutricula is the first known metazoan that has been observed to sexually mature and return to its juvenile colonial stage. This regression from medusa to polyp has only been observed with the presence of differentiated cells from the outer umbrella and part of the animals digestion system. The ability of transdifferention, a non-stem cell which can morph into a different type of cell, in these cells is pivotal for this species' changing life cycle. It is unknown whether or not stem cells play a role in this process. Due to this regular transformation by T. nutricula, it is thought to have an indefinite life span.

References