Turritopsis dohrnii is a species of jellyfish found in Mediterranean Sea and also in Sea of Japan.
It is also known as the immortal jellyfish because it is biologically immortal. This is because the jelly fish once sexually matured can return to its sexually immature form.
It begins its life as a tiny larve which settles on sea bottom. They form a colony of polyps a jellyfish then buds off to form individual jellyfish & become sexually matured.
However if it experience any stress it reverts to polyp stage. This process goes on indefinitely because of this jellyfish is termed as biologically immortal.
However many of the jellyfish can die in nature due to predation or disease without being converted to polyp form.