There may be some truth hidden in the classic American novel, The Fourteenth Goldfish. The fictional coming-of-age novel follows the story of a boy whose grandfather discovers a jellyfish and uses it to regress his age back to an elementary schooler. Though it may seem far-fetched that any being could rewind the biological clock, it isn’t as unusual as it may seem. The Turritopsis dohrnii, a small bell-shaped jellyfish inhabiting temperate and tropic waters, is considered biologically immortal—unable to die of old age. The jellyfish is still prone to poison, predation, and other dangers present to sea life, but its  ability to return itself to a previous state is what makes it immune to aging with time. 

First, we must understand the standard life cycle of the jellyfish. Normally, a fertilized zygote becomes a planula larva in a hydroid colony, one in a network of interconnected larvae. From here, the individual jellyfish matures into a polyp, which buds off in a small blob that is still fixed to the few hard surfaces in the ocean. Then, it becomes a juvenile medusa before reaching the final adult medusa. The jellyfish are then able to reproduce and continue their standard lifestyle.¹ However, its life cycle can also take a completely different turn when encountering danger. After it reaches the reproductive medusa state and is threatened, it retreats in on itself. By forming a small blob, it loses the ability to swim before sinking back down to the ocean floor. Here, it is able to transform back into a polyp through transdifferentiation—the key to biological immortality. 

Transdifferentiation is the process through which the already adult cells in the jellyfish can specialize as new types of mature cells without the need to be reduced to the pluripotent state. In the pluripotent state, the cell can transform into multiple types of cells. This is an incredibly unique ability, as most cells in the body cannot change specialties easily. Cell fate, once determined, lasts forever. Even though recent developments in regenerative medicine have been able to reduce adult body cells (somatic cells) to a pluripotent state, this research is still a step behind the jellyfish.² In stem cell technology, special transcription factors andvarious proteins must be introduced to the cell in order for the proper differentiation to occur, whereas jellyfish cells can skip the pluripotent state. The special ability to reprogram cells to perform vastly different functions allows them to, in a perfect world, continually regenerate themselves. This polyp can then continue to mature, just like the first time, into the fully grown medusa state.³

However, this isn’t just fiction; scientists can consistently reproduce the immortal nature of jellyfish in the lab. Dr. Stefano Piriano and his colleagues in Italy were able to map out the life cycle of the jellyfish as well as the anatomy and physiology of the Turritopsis dohrnii. They also confirmed that the jellyfish did indeed create changes in its DNA expression as it changed between different stages.¹ This suggests that something in its control system allows the jellyfish to manually change what genes are expressed, potentially what allows it to regress in state. While it still isn’t completely clear what molecular mechanisms allow the transdifferentiation of the jellyfish, it is an interesting look at the biological wonders of nature and signals to the untapped potential for medical innovation. 

References

1. Piraino, S., Boero, F., Aeschbach, B., & Schmid, V. (1996). Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa). The Biological Bulletin, 190(3), 302–312. https://doi.org/10.2307/1543022
2. Mollinari, C., Zhao, J., Lupacchini, L. et al. Transdifferentiation: a new promise for neurodegenerative diseases. Cell Death Dis 9, 830 (2018). https://doi.org/10.1038/s41419-018-0891-4
3. Matsumoto, Y., Piraino, S., & Miglietta, M. P. (2019). Transcriptome Characterization of Reverse Development in Turritopsis dohrnii (Hydrozoa, Cnidaria). G3 (Bethesda, Md.), 9(12), 4127–4138. https://doi.org/10.1534/g3.119.400487

Image References

1. Immortality | Philosophy, Religion & Mythology | Britannica. (2025, March 13). https://www.britannica.com/animal/immortal-jellyfish