The Cycle of Rebirth
The hydrozoan Turritopsis dohrnii has a life cycle that defies the usual one-way path from birth to death. This animal, which reaches a maximum size of about 4.5 millimeters, can revert from its adult swimming form, called a medusa, back to its earliest stage, a stationary polyp. This process makes it biologically immortal.
Its life begins conventionally as a fertilized egg that grows into a free-swimming larva called a planula. The planula settles on the seafloor and develops into a colony of polyps, which look like small, stalk-like creatures attached to a surface. These polyps then asexually bud off tiny, genetically identical medusae that swim away to mature. The adult medusa is bell-shaped with a distinct red stomach visible through its translucent body. A young specimen starts with just eight tentacles, but an adult can have up to 90.
Under conditions of stress, such as starvation, injury, or changes in water temperature—the adult medusa can initiate a transformation. Instead of dying, it reabsorbs its tentacles and bell, sinking to the seafloor as a small, blob-like cyst. Within 24 to 36 hours, this cyst develops back into a polyp colony, and the entire life cycle begins anew.
A Cellular Reset Button
The cause of this age reversal is a rare cellular process called transdifferentiation. This allows a specialized, mature cell to transform directly into a different type of specialized cell. For example, a muscle cell from the medusa's bell can become a nerve cell or even a sperm or egg cell in the new polyp. This complete reprogramming allows the organism to rebuild its body from a different life stage.
Turritopsis dohrnii was first described in 1883, but its ability to reverse its life cycle was only discovered by accident in the 1980s by German marine biology student Christian Sommer. Originally native to the Mediterranean Sea, the species has now spread to oceans worldwide. Scientists believe it hitchhiked across the globe in the ballast water of cargo ships, its regenerative ability allowing it to survive long voyages. This has been described as a "worldwide silent invasion" because the jellyfish are tiny and have no discernible negative impact on their new environments.
Despite its biological immortality, the jellyfish is not invincible. In nature, most individuals die from predation by animals like sea slugs, tuna, sharks, and other jellyfish, or from diseases. Its regenerative abilities are a survival mechanism, does not protect against external threats.