The lifecycle of the jellyfish is quite remarkable (pictured below). A jellyfish larva first develops into a polyp and it is this polyp that will then asexually produce thousands of jellyfish. This surely is rather bizarre but thanks to Turritopsis nutricula, this lifecycle gets a whole lot weirder.
Turritopsis nutricula is potentially immortal, it is a hydrozoan whose medusa, or jellyfish, form can revert back to the polyp stage after becoming sexually mature. It is the only known case of a metazoan 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. Cell transdifferentiation is when the jellyfish "alters the differentiated state of the cell and transforms it into a new cell". In this process the medusa of the immortal jellyfish is transformed into the polyps of a new polyp colony. First, the umbrella reverts itself and then the tentacles and mesoglea get resorbed. The reverted medusa then attaches itself to the substrate by the end that had been at the opposite end of the umbrella and starts giving rise to new polyps to form the new colony. Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal.