Turritopsis dohrnii (the immortal jellyfish) belong to the phylum Cnidaria along with other jellyfish, corals, anemones, and sea pens. The distinguishing feature of the phylum Cnidaria is cnidocytes, stinging cells that play a role in capturing prey and providing protection from predators. Among the phylum Cnidaria, the immortal jellyfish belongs to the class Hydrozoa. Hydrozoa’s distinctive feature is a complex life cycle that alternates between the benthic polyp and the planktonic medusa (jellyfish). Turritopsis dohrnii adds unique features to this already complex life cycle. 

Repeating rejuvenation of Turritopsis sp. from medusa to polyp was observed 10 times, which is the most numerous number of such a reversal life cycle. Starting from a single immature medusa collected from Okinawa Island, southern Japan in May 2009 as a founder, every rejuvenated hydroid colony was continuously cultured for about two years, being kept separately in respective tank filled with fresh natural seawater (32 psu) at Shirahama, Wakayama Prefecture, Japan in a running seawater system in the laboratory at 21-29 ℃ without strong natural sunlight which may induce algal growth on rearing vessels, and being fed mainly with Artemia nauplii.   C