A species of small, biologically immortal jellyfish named by German biologist August Weismann in 1883. The immortal Jellyfish (or Turritopsis dohrnii) is believed to have originated in the Pacific and to have spread worldwide through migrations. It is now found free-living in the plankton in temperate to tropical regions in all of the world’s oceans.
The immortal jellyfish is bell-shaped, with a maximum diameter of about 4.5mm (0.18in) and is about as tall as it is wide. Young specimens (1mm/0.04in in diameter) have only eight tentacles evenly spaced out along the edge, whereas adult specimens have 80–90 tentacles.
It is the only medusa known to have developed the ability to return to a polyp state, by a rapid specific transformation process. The process has not been observed in their natural habitat but experiments have revealed that all stages of the medusae, from newly released to fully mature individuals, can transform back into polyps under the conditions of starvation, sudden temperature change, reduction of salinity and artificial damage of the bell with forceps or scissors. This ability to reverse the biotic cycle in response to adverse conditions is unique in the animal kingdom. It allows the jellyfish to bypass death, rendering Turritopsis dohrnii potentially biologically immortal. The species’ 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.