Immortal Jellyfish

One of the many features of video and computer games is the save or restart button. Make a mistake, fall down an abyss, lose a battle, and you can go anew at a previous game state. The real world doesn’t yet work that way for animate beings.

Unless you’re one of a few specific types of jellyfish.

Consider Turritopsis dohrnii:

Turritopsis dohrnii - photo by Bachware

Most jellyfish are not long for the planet. Depending on the species, they typically live for mere hours, days, or months. 

This species, however, is known as the immortal jellyfish. Along with Turritopsis nutricula, Laodicea undulate, members of the genus Aurelia. and perhaps a few others, these jellyfish defy all we currently understand about aging. They display a trait known as biological immortality. The definition of this term is important: the state in which mortality from senescence is stable or decreasing. Layperson translation: the mortality rate does not increase as the organism ages. Put another way, getting old has nothing to do with dying for these jellyfish. Biological immortality does not equate to invulnerability. Predation, disease, injury, poison, or other environmental factors can still kill these jellyfish, they are just unlikely to die because they age.

The mystery of aging seems to be locked to cell division. Human cells can divide 40 to 60 times before they reach the senescence phase. Scientists call this the Hayfleck limit. We currently cannot keep this process from transpiring; at some point, our cells stop dividing. In a biologically immortal entity, however, this problem does not exist.

How does it work? Jellyfish are a little like butterflies. They have distinct morphological cycles.

The jellyfish life cycle - graphic by Smithsonian Ocean Portal

Parent jellyfish spawn larvae known as planulae. These proto-jellyfish swim around for a while before attaching themselves to the ocean floor. There, they become a colony of polyps, looking like gelatinous forests on the sea bed. Every polyp that comes from a single planula is a genetic clone. Eventually, the polyps transform into the adult form, known as the medusa. The medusae grow, bobbing around the world’s oceans, until they become sexually mature, at which point they drop planulae and the circle of life continues.

For the immortal jellyfish, this cycle does not need multiple individuals to continue.

Suppose Turritopsis dohrnii encounters a big problem, such as a hungry predator, a lack of resources, or a plague, and it survives the encounter in a potentially fatal state. Here, the jellyfish becomes a biological magician. Through a process called transdifferentiation, the immortal jellyfish can revisit the polyp stage of its life cycle. Essentially, its somatic cells (i.e. non-sexual cells) program themselves to revert to a point younger than its current state. The polyps grow, just like they did previously, into a medusa.

New lease on life!

Turritopsis nutricula medusae - photo by Totti

In theory, the immortal jellyfish could continue this regeneration forever. Obviously, we have not witnessed an individual living for eternity, but the process can seemingly continue ad infinitum. Most members of these species likely do perish, as they are munched by something else; some succumb to disease, despite the superpower.

If we could unlock the capability of the immortal jellyfish, we could potentially solve the mystery of cell division and aging. Scientists have a massive interest in Turritopsis dohrnii et al. for glaring reasons. One of the cutting edges of science at the moment involves stem cells. Normal cells are specialized. Whether carrying oxygen (red blood cells) or moving body parts (muscle cells), they carry out a specialty, they can replicate into other cells of their type, and they eventually cannot divide anymore. Stem cells, on the other hand, can develop into many different types of cells and they can divide seemingly without limit. Stem cells have the medicinal potential to fix many problems that other solutions cannot, thanks to their flexibility. Some scientists believe stem cells hold the key to solving the mystery of aging and, perhaps, death and disease.

Is the ultimate resolution to stem cells and aging on display in the immortal jellyfish? Many researchers believe the answer is yes. If we could understand how the jellyfish transdifferentiate, perhaps we can open the superpower to our own biology.

Image of an adult medusa of the Turritopsis nutricula species - photo by Alvaro E. Migotto

Unfortunately, rearing the immortal jellyfish in captivity has proven rather arduous. A scientist at Kyoto University managed to nurse a colony for two years, during which they rebirthed 11 times. Shin Kubota loves his immortal jellyfish so much that he wrote songs about them to perform during his lectures.

Large-scale experiments on the tiny creatures have, so far, not yielded much success, though.

The immortal jellyfish also raises a philosophical question.

If an individual’s cells revert to the polyp stage and later become a new medusa, is that specimen still the same individual? From the standpoint of genetic material, the jellyfish might seem identical. However, clones are genetically identical entities. Are clones all the same individual or different? If a human could beat a physical problem by reverting to the childhood stage, would that human retain whatever it is that makes an individual? Is memory and consciousness necessary for individuality? Would an entity that undergoes transdifferentiation be able to retain memory? Or would the entire process render a new individual? After all, the mass in our bodies is the remnant of starstuff from the distant past. One could argue we are therefore stars, yet we do not retain the memory of being part of celestial dynamos. Are the atoms – our physical makeup – what constitutes the individual or is something more at play? Is the spark we sense within us real or just an illusion? Does the alignment of atoms matter? If it does matter, can we rearrange them and still somehow come out as ourselves?

The answers to these questions – if any answers exist – are farther away than even the inner workings of stem cells. Perhaps, in the future, we might reverse aging and cure disease, putting ourselves in the position to answer such queries. If we do so, we might follow the sea-road map produced by immortal jellyfish!

Further Reading and Exploration

The Immortal Jellyfish – American Museum of Natural History

Immortal jellyfish: the secret to cheating death – U.K. Natural History Museum

The secrets of the immortal jellyfish, Earth’s longest-living animal – BBC Science Focus

Everlasting life: the ‘immortal’ jellyfish – The Biologist

Repeating rejuvenation in Turritopsis, an immortal hydrozoan by Shin Kubota

JELLYFISH AND COMB JELLIES – Smithsonian Ocean Portal

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