The Immortal Jellyfish: An Animal That Restarts Its Life

Most animals follow a strict one-way life cycle: born, grow, reproduce, age, die. Turritopsis dohrnii, a jellyfish measuring only about 4.5 mm across, does not. When it is starving, injured, or environmentally stressed, its bell-shaped medusa body sinks to the sea floor and its cells transform back into a colonial polyp — the earliest post-embryonic stage of its life. From that polyp, new medusae bud off, genetically identical to the original.

The mechanism is called transdifferentiation. Cells that had already specialized as, for example, muscle cells, are able to switch identity and become nerve cells, egg cells, or something else entirely. In humans this only happens under strict, limited conditions like liver regeneration. In Turritopsis dohrnii it is a whole-body response, effectively rebooting the animal's development.

This does not mean the individual jellyfish is invincible. Immortal is a biological term, not a practical one — the animal is still easily eaten by predators or killed by disease. But barring accidents, there is no biological ceiling on its lifespan. In principle, a single genetic line can persist indefinitely by cycling between polyp and medusa forms.

Turritopsis dohrnii was originally described in the Mediterranean but has spread worldwide, most likely by hitchhiking in the ballast water of cargo ships. That makes it one of the very few invasive species that carries its own reset button — a survival strategy that seems tailor-made for stressful, unfamiliar environments.