There is a sort of jellyfish which is the most amazing creature you will ever heard about, and that's because it developed a characteristic so special that no other creature in the animal world, not even other jellyfishes very closely related to it, developed. It is immortal.
It could seems unbelievable but it's just simple as that, this Jellyfish never dies for natural reasons (obviously it can be killed)
This incredible creature is the Turritopsis nutricula, it's not exactly one of the jellyfishes we're used to (Cubozoa and Scyphozoa), infact it is an Hydrozoa a taxonomic class that is part of the Phylum target="_blank"Cnidaria with Jellyfishes (Cubozoa and Scyphozoa) and corals (Anthozoa). What is curious is that Hydrozoans share a parte of their life as corals and another one as jellyfishes, to be more precise they have a unusual life cycle in which they change from a polyp stage (coral) colonial and rooted, to a medusa stage (jellyfish) solitary and floating. This is an extreme semplification of a reality which is much more complicated and wide-ranging, but it's enough to understand better our "Highlander" jellyfish. So let's go back to our Turritopsis nutricula, what makes it so unbelievably special is that it is able to revert its life cycle from medusa to the polyp stage after becoming sexually mature. It is the only known case of an animal capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage. It does this through the cell development process of transdifferentiation. Theoretically, this cycle can repeat indefinitely, rendering it effectively immortal.
Don't let the pictures mislead you, our little immortal has a diameter of just 4-5 millimeters just big enough to see with your naked eye. Young specimens have only eight tentacles along the edge (see picture above), while adult specimens have 80-90 tentacles (see pictures below).