Are Jellyfish Immortal?

Growing old is usually a fairly one-directional affair. You’re born, you grow, you get older, and eventually your cells start behaving like they have collectively decided to resign. There is no undo button. There is certainly no going back to being younger.

Unless you are Turritopsis dohrnii, a jellyfish so committed to avoiding death that it can, under certain conditions, essentially restart its life cycle.

The species has earned the rather dramatic nickname “the immortal jellyfish” because an adult medusa can reverse its development and return to its juvenile polyp stage. When damaged, stressed or aging, it can shrink, lose its adult form and transform through an intermediate cyst stage before becoming a polyp again. From there, it can produce new medusae. The process can, in principle, happen repeatedly.

This is not immortality in the way humans usually imagine it. The jellyfish can still be eaten. It can still get sick. It can still die. What makes it unusual is that aging does not necessarily have to be the end of its life cycle.

Basically, while the rest of us are stuck watching the progress bar move from childhood to adulthood to whatever mysterious stage comes after 25, this jellyfish has discovered Ctrl+Z.

To understand why this is so strange, you have to understand that jellyfish do not begin life looking like the floating, translucent blobs we picture, when someone says “jellyfish.”

T. dohrnii begins as a larva called a planula. It eventually settles onto a surface and develops into a polyp: a small, attached form that produces young jellyfish. Those jellyfish, called medusae, grow, mature and reproduce.

Normally, this is where the story ends. But, for T. dohrnii, apparently, it does not.

If an adult medusa is physically damaged or exposed to unfavourable conditions, it can undergo life-cycle reversal. Its body shrinks and loses the structures that make it recognisably adult. It then enters a cyst-like stage before developing back into a polyp. Researchers have observed the cyst beginning

to develop polyp-like structures within roughly 24 to 36 hours.

It is not healing in the conventional sense. It is more like the jellyfish looks at its entire adult body and says, actually, never mind. The distinction matters because the animal is not simply repairing an injured tentacle or replacing a damaged cell. It is reversing its developmental trajectory. The biological trick behind this is called transdifferentiation.

Cells in an adult body are usually specialised. A muscle cell does not wake up one morning and decide it would rather be a skin cell. Specialisation is one of the basic organising principles of multicellular life.

Transdifferentiation is different. It involves a mature, specialised cell changing into another cell type. In T. dohrnii, researchers think this cellular flexibility is part of what allows the adult medusa to reorganise itself into the much earlier polyp stage.

That is what makes the jellyfish particularly interesting to scientists studying aging and regeneration.

It is not simply keeping its body young - it is changing what its body is.

Researchers studying the species have found that the cyst stage involved in reversal shows changes in gene activity associated with processes including DNA repair, aging, telomere maintenance and cellular reprogramming. A 2022 comparative-genomics study also identified differences between T. dohrnii and the related Turritopsis rubra, which does not show the same repeated rejuvenation after maturity.

That comparison is important. If two closely related animals have different abilities to reverse their life cycles, their differences may offer clues about the biological machinery involved.

And suddenly, the tiny jellyfish floating around in the ocean becomes a rather useful research subject.

The word “immortal” carries a lot of weight here. It is also slightly misleading.

Turritopsis dohrnii is not an underwater Highlander. It does not float around the Mediterranean, eternally avoiding death while waiting for its next villain. Predators can eat it. Disease can kill it. Environmental changes can kill it. The species' unusual ability is specifically about reversing its development and potentially escaping death associated with aging or severe stress. Scientists therefore use the idea of biological immortality rather than suggesting the animal is literally impossible to kill.

That distinction is particularly important because the jellyfish's ability has sometimes been stretched

into headlines suggesting that humans are about to become immortal.

We are not.

A jellyfish reverting to a polyp is not the first step in a human anti-aging treatment. Human bodies have vastly more complex tissues, organs and regulatory systems. The fact that a jellyfish can reprogramme cells does not mean a human can simply do the same.

What the jellyfish can do is give researchers a very strange but useful system for asking questions about aging. And science, occasionally, starts with a question that sounds slightly ridiculous - What does the jellyfish know about getting old?

Aging is not just about becoming visibly older.

At the cellular level, organisms accumulate damage and experience changes in processes such as DNA maintenance, cellular communication and the ability of cells to divide and function properly. Understanding how some organisms deal with these processes differently could help researchers understand why aging happens in the first place.

This is where T. dohrnii becomes more than a fun fact for a marine biology quiz.

The 2022 genome study identified differences in genes and gene networks associated with DNA repair, telomere maintenance, replication, redox processes, stem-cell populations and cellular communication. During life-cycle reversal, the researchers also found changes linked to pluripotency - the capacity of cells to develop into different cell types.

None of this amounts to a human fountain of youth, but it does give researchers somewhere unusual to look.

The species is being studied as a model for understanding cellular plasticity, regeneration and aging. Researchers are interested in how its cells change identity, how its tissues reorganise themselves and what genetic networks switch on and off during the process.

The eventual significance may therefore have less to do with making people live forever and more to do with understanding how cells repair, replace and reorganise themselves.

Which is considerably less exciting than “scientists discover immortality,” but considerably more useful.

But there is something almost offensive about Turritopsis dohrnii.

Humans have spent centuries trying to negotiate with aging. We have invented anti-aging creams, supplements, procedures, diets and an alarming number of products that promise to make you look “ten years younger” by Tuesday.

The jellyfish does none of this - it just changes its life cycle. And that is what makes it scientifically fascinating. 

T. dohrnii has not conquered death. It has evolved a way to move backwards developmentally when death might otherwise be the end of the story. Its cells appear capable of an extraordinary amount of reorganisation, allowing the animal to return from an adult medusa to a juvenile polyp and begin the cycle again.

Perhaps that is what makes the T. dohrnii so interesting. It does not show us how to live forever. It shows us that, in biology, growing old is not always as irreversible as we assume.

For one very small jellyfish, the end of the story can become the beginning again.