The Immortal Jellyfish
It’s one thing to survive in harsh environments, but quite another to hit the reset button when faced with an imminent threat. Only one animal is known to have this remarkable ability: a species of jellyfish, Turritopsis dohrnii, first discovered in the 1880s in the Mediterranean Sea and highlighted as a uniquely enduring organism in the exhibition Life at the Limits: Stories of Amazing Species.
Like all jellyfish, Turritopsis dohrnii begins life as a larva, called a planula, which develops from a fertilized egg. A planula swims at first, then settles on the sea floor and grows into a cylindrical colony of polyps. These ultimately spawn free-swimming, genetically identical medusae—the animals we recognize as jellyfish—which grow to adulthood in a matter of weeks.
Fully grown, Turritopsis dohrnii is only about 4.5 mm (0.18 inches) across, smaller than a pinky nail. A bright-red stomach is visible in the middle of its transparent bell, and the edges are lined with up to 90 white tentacles. These tiny, transparent creatures have an extraordinary survival skill, though. In response to physical damage or even starvation, they take a leap back in their development process, transforming back into a polyp. In a process that looks remarkably like immortality, the born-again polyp colony eventually buds and releases medusae that are genetically identical to the injured adult. In fact, since this phenomenon was first observed in the 1990s, the species has come to be called “the immortal jellyfish.”
The cellular mechanism behind it—a rare process known as transdifferentiation—is of particular interest to scientists for its potential applications in medicine. By undergoing transdifferentiation, an adult cell, one that is specialized for a particular tissue, can become an entirely different type of specialized cell. It’s an efficient way of cell recycling and an important area of study in stem cell research that could help scientists replace cells that have been damaged by disease.
As for Turritopsis dohrnii, this jelly is not only an extraordinary survivor. It’s also an increasingly aggressive invader. Marine species have long been known to hitch rides around the world in the ballasts of ships. Researchers have recently identified the immortal jellyfish as an “excellent hitchhiker,” particularly well-suited to surviving long trips on cargo ships.
In the same study, researchers also documented essentially genetically identical Turritopsis dohrnii individuals distributed across the world’s oceans, raising an intriguing question about the nature of mortality—if all of an organism’s cells are replaced, is it still the same individual? The genes are the same, of course—and in biology, that may be enough to declare a winner.