The natural ageing process is perceived as unidirectional. Ageing occurs for all living organisms, from animals and plants to humans, and involves the gradual deterioration of cells' functionality and loss of capacity for self-repair. But there was an animal found that challenged all previous notions concerning the ageing of living creatures on Earth. The immortality jellyfish, otherwise referred to by its scientific name Turritopsis dohrnii, has the unique ability to revert to an earlier developmental state following stress, illness or hunger. Transdifferentiation is what makes this creature an intriguing object of study for marine biologists and scientists working on the longevity of human cells, stem cells, etc.What is the immortal jellyfish, and why are marine biologists amazedThe immortal jellyfish is a tiny translucent jellyfish that inhabits the oceans worldwide and especially thrives in tropical and temperate seas. Despite being small and only a few millimetres in size, it is arguably one of the most important discoveries made in the field of marine biology.Contrary to the usual pattern in the natural world, where organisms undergo birth and eventually die after a certain period of life, Turritopsis dohrnii exhibits a unique ability. When the jellyfish finds itself in stressful conditions such as starvation, injury, or adverse environmental effects, it is capable of turning itself into the juvenile form called polyps. Thereafter, the animal starts a new life cycle.The first scientists who came up with the idea that Turritopsis dohrnii exhibits such an interesting phenomenon did so back in the 1990s. The publication titled 'Complete mitochondrial genome and evolutionary analysis of Turritopsis dohrnii, the "immortal" jellyfish with a reversible life-cycle' that appeared in the journal Molecular Phylogenetics and Evolution noted that this jellyfish was “the only known metazoan that is capable of reversing its life cycle via morph rejuvenation from the adult medusa stage to the juvenile polyp stage”.While being immortal in theory, this creature cannot escape death. However, its capacity to repeatedly reset its biological clock makes it unique in the animal kingdom.How does this jellyfish reverse its ageing processJellyfish's longevity and the incredible rejuvenating abilities of the species can be attributed to transdifferentiation, a biological phenomenon in which mature cells can develop into other kinds of cells.Unlike other organisms, where specialised cells remain constant in function throughout an animal's life, in Turritopsis dohrnii, adult cells can reverse development and reprogramme themselves into more juvenile states. This enables the animal to regenerate its body and go back in time to its youth.Studying jellyfish genomes, scientists managed to isolate the genes associated with DNA damage repair, stem cell regulation, and cellular regeneration. It was revealed during the research carried out by specialists from the Kazusa DNA Research Institute that such genes might play an important role in jellyfish rejuvenation.A scientist named Shin Kubota, specialising in marine biology, observed the organism's capacity to rejuvenate in the laboratory environment during his years-long research on Turritopsis dohrnii. The observations made by the scientist have led to a greater understanding of what triggers cell reversal.The species can potentially become an incredibly important model for research in the field of cell regeneration and ageing, though many mechanisms of the process are still a mystery to researchers.Why scientists are fascinated by the immortal jellyfishOne of the most significant developments regarding immortality and regeneration has been the emergence of the immortal jellyfish as an object of study in modern longevity and regenerative medicine research.What interests scientists the most about this species is how it restores its body and cells without any loss of biological function. Humans, for instance, grow older mostly due to accumulated damage and changes within cells, including the degradation of repair mechanisms and DNA mutations. It seems that this process can be circumvented in the case of the immortal jellyfish.In a recently published scientific review titled ‘Regenerative characteristics of the immortal jellyfish, Turritopsis dohrnii, and their potential implications for human ageing’, which appeared in PubMed, it was stated that the regenerative abilities of the Turritopsis dohrnii “may have potential implications for human ageing”.The studies related to this topic might contribute to our knowledge about age-related diseases, stem cells, and tissue regeneration.Currently, no scientist is anywhere close to achieving this feat in the human body. But one thing is for sure: this species keeps on changing the way scientists think about ageing.