Turritopsis dohrnii

Turritopsis dohrnii is a hydrozoan cnidarian whose life cycle includes a free-swimming medusa and a polyp stage. Like other jellyfish, it is a small marine invertebrate. The common name “immortal jellyfish” refers to its unusual ability to reverse its life cycle, not to immunity from all mortality. The Natural History Museum describes the animal as only a few millimetres in size.

When an adult medusa is physically damaged or exposed to stresses such as starvation, it can shrink, lose its swimming ability and settle as a cyst. The cyst can then develop into a polyp, an earlier life stage, from which new medusae can bud. This reversal involves cellular reorganization, including transdifferentiation, in which specialized cells change into other cell types. Under suitable conditions, the sequence can be repeated.

Repeated rejuvenation has led researchers to describe T. dohrnii as biologically immortal in a qualified sense: it can avoid the usual one-way progression from adult to death by reverting to an earlier stage. It can still die from external causes, including predation and environmental harm, and its long-term survival in nature is not guaranteed. Research has examined gene expression during life-cycle reversal and compared its genome with that of the related species Turritopsis rubra, which has not shown the same post-reproductive rejuvenation. These studies investigate possible mechanisms; they do not establish a method for preventing ageing in humans.