Which animal has a subspecies that's considered to be biologically immortal?
And the answer: jellyfish.
Also known as the immortal jellyfish, the Turritopsis dohrnii is a species of small jellyfish found in warm waters around the world. When exposed to stress or illness, it can revert back to an earlier stage, and grow through its life stages again, theoretically continuing this cycle for eternity.
This tiny jellyfish is unequivocally more prepared than many of its jelly relatives. Where every other species of jellyfish awaits post-reproduction death (if they have not otherwise been eaten by some other organism), the Turritopsis dohrnii becomes effectively immortal because of its ability to transform from the medusa stage (fully formed jellyfish) back to the polyp stage (near equivalent to a tadpole), thereby escaping death.
Though jellyfish populate nearly every saltwater congregation (and even some freshwater) on this planet, their life cycles are complex and poorly understood. These creatures are ancient, dating back millions of years before the dinosaurs. Interestingly, their physiology is remarkably similar to their prehistoric ancestors', but the wide variation of form and unpredictable survival patterns make it difficult to study these jellies of the sea.
Jellies can vary greatly in size. From those measuring in at just one cubic centimeter across, to those with up to 7 feet in diameter (plus 120 feet in length!), it's no understatement to say that jellyfish come in all shapes and sizes. Ironically, the smaller the jellies are, the more venomous they can be. The smallest jellyfish known to man, the Irukandji jellyfish, packs a punch 100 times as potent as that of a cobra and 1,000 times stronger than a tarantula's. And, as if that wasn't enough, check out this photo of the lion's mane jellyfish (the biggest jelly documented to date):
Though they have no brain, heart or eyes, and are composed mainly of water, these creatures of the sea are some of the most magnificent and fascinating creatures on the planet.
Check out this Nat Geo video for more: