If you ever went for a swim at Australia’s east coast, you might have encountered, among many different marine species, a small jellyfish. Not the kind that has the potential to inflict upon you a most painful and untimely death, no; this species is entirely harmless. Translucent and only around five millimetres in length, it probably did not even draw your attention, and so it would not have occurred to you that you just encountered one of the most extraordinary miracles on the planet – an immortal being.

Since the dawn of time, man has dreamed of to tricking death and gaining immortality. Religions have formed around the promise of eternal life and it serves as a ubiquitous motive in music and fiction, ranging from ancient Greek mythology to Pirates of the Caribbean and Doctor Who.
Alas, despite well-funded scientific efforts and vast improvements in our quality of life, we have only managed to extend our average lifespan by a couple of decades. Immortality, it seems, is out of reach for the foreseeable future, if not forever. But what applies to humans does not necessarily hold true for other forms of life. Several kinds of bacteria are thought to be immortal and animals such as lobsters and hydras do not seem to age the way other creatures do. Lobsters, for example, simply get bigger and bigger the older they grow and, surprisingly, more fertile.

If an ‘immortal’ polyp hydra is cut in half, both halves will grow into full hydras

In 1988, young German biology student Christian Sommer went snorkelling at the Italian coast. Looking for hydrozoans for his research project, he accidentally picked up a specimen of Turritopsis dohrnii, a species of tiny jellyfish. Keeping his catches in petri dishes, he soon noticed something strange: the jellyfish neither aged nor died.
The life cycle of your average jellyfish consists of several stages. A colony of polyps, firmly attached to solid ground and looking similar to corals, produces medusae, the stage of jellyfish we see swimming around in the ocean. This in turn produces eggs and sperm which turn into an embryo and settle on the ocean floor or in a reef where they form a new polyp – the adult medusa dies.

Turritopsis dohrnii

T. dohrnii, however, has managed to bypass this cycle by resorting to a process called transdifferentiation. Several species of animals are known to turn mature cells back into stem cells in order to produce another type of mature cells, a process called dedifferentiation, but to date T. dohrnii is the only known instance of an organism entirely skipping the stem cell stage and directly turning mature cells into other cell types. It does not die after reproduction, but simply flips its umbrella inside out, absorbs its own tentacles and turns itself into a polyp again, essentially reversing its life cycle. Shortly thereafter it spawns and produces another medusa consisting of the same cells, therefore effectively being the same organism. Shin Kubota, a Japanese biologist and the only known person managing to keep specimens alive in captivity over a prolonged period of time, reported that within a period of two years, his jellyfish completely regenerated themselves ten times. The process occurs naturally, but can also be provoked by stabbing a medusa repeatedly, thus forcing it to regenerate. Biological immortality does not translate into eternal life though. T. dohrnii can still be killed, and in fact most are eaten by predators sooner rather than later.

Whether research into the species will one day lead to immortality for humans is highly questionable, and while Shin Kubota believes that it holds great potential, most of the scientific community is less optimistic. For the time being the mere knowledge that immortal life exists should be exciting enough.