Turritopsis dohrnii, the immortal jellyfish, is a species of small, biologically immortal jellyfish native to the Mediterranean Sea and the waters of Japan
The immortal jellyfish's top 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 jellyfish are 1 mm in diameter and have only eight tentacles evenly spaced out along the edge, while adults have 80–90 tentacles.
They eat plankton, tiny molluscs, larvae and fish eggs.
They are the only animal in the world that can not die of old age. Although they can be fall to disease or be eaten.
The Immortal Jellyfish has two distinct stages: The polypoid stage(baby) and the medusa stage(adult).The jellyfish will reach its sexual maturity in less than 30 days if the water temperature is 20 degrees Celius or in 18-22 days if the water temperature is 22 degrees.
It turns out that once the adult form of the 4.5 mm-wide jellyfish reproduce, they don’t die but transform themselves back into their juvenile polyp state. Their tentacles retract, their bodies shrink, and they sink to the ocean floor and start the cycle all over again.The immortal jellyfish regularly undergos this change.
The only known way they can die is if they get consumed by another fish or if a disease strikes the jelly. However, there are still many mysteries surrounding the species. While the process of reverting from its adult-phase to a polyp was observed several times, it hasn’t been observed yet in nature, only in laboratory environments.
In other words it is a jellyfish that when injured or dying starts it life cycle over again.
Because they are 'immortal' they are not in danger of going extinct.
It's scientific name is Turritopsis dohrnii