The immortal jellyfish as described on Wikipedia:

Jellyfish usually die after propagating; however, the Turritopsis nutricula has developed the ability to return to a polyp state. This is done through a cell change in the external screen (exumbrella). The cells revert to a different state. The medusa is transformed into a stolon and the polyps into a hydroid colony.^[3] The umbrella turns inside out; middle section and tentacles are reabsorbed before the polyp spawns. Stolons form two days before the polyps differentiate.^[3] The ability to reverse the life cycle is probably unique in the animal kingdom, and allows the jellyfish to bypass death, rendering the Turritopsis nutricula biologically immortal. Lab tests showed that 100% of specimens reverted to the polyp stage.