Which jellyfish is often described as biologically immortal?

Turritopsis dohrnii is the jellyfish often described as biologically immortal.

This tiny hydrozoan can reverse its life cycle after injury, starvation or other stress. Instead of continuing from the adult medusa stage toward death, it can transform its cells and return to a juvenile polyp stage. The polyp can then produce new medusae, creating a potential cycle of rejuvenation.

The process is called transdifferentiation, in which one mature cell type changes into another. It does not make the animal impossible to kill: predators, disease and environmental damage can still end an individual. “Biologically immortal” therefore refers to the possibility of avoiding normal aging under suitable conditions, not to invulnerability.

Turritopsis dohrnii was formerly grouped with the related species Turritopsis nutricula, which caused confusion in popular accounts. The species is very small and difficult to identify in the wild, and its remarkable ability does not mean that every individual successfully completes repeated life-cycle reversals.