The immortal jellyfish (Turritopsis dohrnii) is a species of jellyfish with the ability to biologically be immortal. It is one of the only animals on earth with the ability to revert back to the younger stage of its life.
Like most jellyfish, they start as a larva known as a planula. When a planula reaches the ocean floor, it creates a colony of polyps that are attached to the seafloor. Eventually, the polyps break off of the colony and now can pulse its “bell” to move. The polyp eventually grows into a full grown adult (medusa), but for the immortal jellyfish, when it becomes sick or it is dying, it will revert back into a polyp colony. This process could go on forever making the immortal jellyfish biologically immortal.
The medusa of the immortal jellyfish is bell-shaped, with a maximum diameter of about 4.5mm. The jelly in the walls of the bell is thin, except for some thickening at the apex. The relatively large stomach is bright red and has a shape in cross section. Young specimens 1mm in diameter have only eight tentacles evenly spaced out along the edge, whereas adult specimens have 80–90 tentacles. The jellyfish is free-living in the plankton. Dense nerve nets also present in the mesoglea (skin) in the cap.
Keeping the immortal jellyfish in captivity is quite difficult. As of now, only one scientist, Shin Kubota, has managed to sustain a group of these jellyfish for a prolonged period of time. The plankton must be inspected daily to ensure that they have properly digested the Artemia cysts they are being fed. Kubota reported that during a two-year period, his colony rebirthed itself eleven times.