What can the jellyfish Turritopsis dohrnii do when it is starved or injured?

Answer

This life-cycle reversal is why it is called the immortal jellyfish, though it can still be eaten or die of disease.

Context

Turritopsis dohrnii is tiny. An adult medusa has a bell only about 4.5 millimetres across, and its polyp colonies reach roughly 35 millimetres. The species is native to the Mediterranean and has since spread to warm coasts worldwide, probably carried in ship ballast water.

The reversal process is called transdifferentiation. When injured or stressed, the medusa's bell contracts, the tentacles deteriorate, and the animal sinks as a small ball of tissue. Its specialised adult cells give way to unspecialised ones, and over roughly 24 to 36 hours these form a polyp that can bud off new, genetically identical medusae. Triggers described include physical damage, low food supply, and changes in salinity or temperature.

The word immortal is therefore a statement about potential, not invincibility. Individuals can still be eaten by fish, sea turtles, sea anemones and other predators, and can die of disease. In the wild, no one has watched a single animal cycle indefinitely, so claims of true immortality are best read as biological immortality in principle.

Researchers study the species' gene activity to understand how cells change identity, and recent papers discuss possible lessons for aging research. Those links to human aging remain speculative, and no therapy has come from them.