The Turritopsis nutricula, right (photo via Boing Boing), a jellyfish found throughout the world, has a unique trait: biological immortality. Turritopsis go through the typical life changes of any jellyfish — fertilized eggs hatch into larvae; the larvae grow to be polyps; in time, the polyps become fully mature jellyfish. But the mature Turritopsis have an additional trick — they can shed their tentacles and bell and then revert back to the polyp stage. In short: instead of getting old and dying due to their advanced age, Turritopsis instead become baby jellyfish and start life over again.
Of course, being this incredible comes with a price — Turritopsis are still, in the end, jellyfish, and tiny ones at that. They grow to be only about a quarter-inch wide and are regularly susceptible to environmental dangers such as being eaten by larger organisms. So while Turritopsis are able to turn back their own biological clocks, they aren’t actually immortal; rather, they’re readily killed in the normal course of ocean events.
From the Archives: Conan the Bacterium: It’s not immortal, but it’s incredibly resistant to radiation.
Related: Magic Jellyfish, or something.