The title does not lie: There really is such a thing as an immortal jellyfish. Turritopsis nutricula does not know the meaning of 'die.' At any stage in its lifetime, it can revert back to its polyp form - the rough equivalent of a mature human becoming a baby again. This effectively makes it immortal.
How does it do this? Well, some creatures, such as salamanders, can regenerate parts via a process called "transdifferentiation." In laymen's terms, one type of non-stem cell becomes another. Usually, this process is irreversible and only occurs in one or two parts of the creature's body.
This jellyfish can reshuffle its whole life cycle. Some little critters without complex brains just flipped science the bird.
Despite the chance for immortality, many of these jellyfish do not live long, eternal lives. They are still susceptible to diseases and predation. They are, however, prolific enough to be overcrowding the oceans.
Now, I know these guys are not cephalopods, but this STILL merits an "IA CTHULHU!" or two. One of those jellyfish, mutated by radiation, waits in the R'lyeh dreaming. It's not dead, just sleeping.