While it is often joked that cats have nine lives, a certain species of jellyfish has been deemed "immortal" by scientists who have observed its ability to, when in crisis, revert its cells to their earliest form and grow anew. That means that these tiny creatures, 4 millimeters to 5 millimeters long, potentially have infinite lives.

Living Forever

The creature, known scientifically as Turritopsis nutricula, was discovered in the Mediterranean Sea in 1883, but its unique regeneration was not known until the mid-1990s. How does the process work? If a mature Turritopsis is threatened — injured or starving, for example — it attaches itself to a surface in warm ocean waters and converts into a blob. From that state, its cells undergo transdifferentiation in which the cells essentially transform into different types of cells. Muscle cells can become sperm or eggs, or nerve cells can change into muscle cells, "revealing a transformation potential unparalleled in the animal kingdom," according to the original study of the species published in 1996.

More Immortal Sea Creatures

Since the Turritopsis’ virtual immortality was discovered, so have swarms of genetically identical jellyfish far from their original habitat, including in Japan, Spain and the Atlantic Ocean side of Panama. Researchers have concluded that these multiplying creatures are getting caught in ballast waters, water that is sucked into and pumped out of the long distance cargo ships. Polyps also could be growing on the ship’s hulls. Though genetically identical, these jellyfish seem to have adapted to their new environments. For example, specimens from swarms living in tropical waters have been found to have eight tentacles, while those discovered in temperate regions have 24 or more tentacles.

But Turritopsis can — and do — die. Their regeneration only occurs after sexual maturation, therefore they can succumb to predators or disease in the polyp stage. But because the jellyfish are the only known animal with this "immortality," scientists are studying them closely, with the hopes of applying what they learn to issues such as human aging and illness.