The hydromedusa of Podocoryna borealis. (Image credit: Lara Beckmann, NorHydro, CC BY-SA 4.0)

Our existences are often centered around the hope that we will live a long and fulfilled life. At the same time, while we aim to grow old, many of us abhor the aging process, dreaming of remaining young and healthy for as long as possible. It explains why we are so fascinated by the concept of immortality. Think of vampire stories, constant quests for the fountain of youth, or even the newest anti-aging products in the drugstore next door. But apart from the few extra years we gain nowadays through modern medicine and improved life circumstances, many of us can’t extend our lives much further.

We share this fate with many other animals that go through the stages of birth, growth, reproduction and death. But despite that, we don’t need to rely on science-fiction to get a glimpse of everlasting life: some organisms on our planet don’t follow these stages and some cheat it altogether – escaping death almost completely.

The Old and Wise 

Some older animal lineages than ours have made some serious inroads into forms of regeneration and even come very close to immortality. One lineage is especially prone for this and is studied heavily because of this feature: the Cnidarians, a phylum which includes the jellyfish and polyps. This group evolved between 700 to 550 million years ago, well before the first animal made its first step on land.

To understand what makes them so unique, we need to have a look at the typical cnidarian life,  which progresses very differently to ours. What we think of as a jellyfish is most often its adult form, called a medusa. When a medusa reproduces, it will release eggs or sperm which will be fertilized and grow to a wormlike larva, called planula larva, which settles on the seafloor. From there it grows either to a single polyp or a polyp colony. Those can spread out asexually and finally will release medusae again. Not all cnidarian species follow these stages strictly and there is a lot of variety of life cycles, but some have even entirely lost the medusa or polyp stage

This might seem different, but we still see stages that humans are familiar with: birth, growth, reproduction and… death? Yes, because the mature medusa will most likely die after the reproductive period. In a polyp colony death is less obvious because during the asexual propagation the genetically identical polyps can grow (theoretically) indefinitely and will be replaced by new ones if necessary. Indeed, if the conditions are good (plenty of food and no predators or diseases) any colonial animal can survive for a long time, however in the long run even the last polyp of a colony will die eventually. 

Some corals, like the gold coral of the genus Savalia (shown on the left) are estimated to reach well over 1,000 years. Deep-sea sponges, as the one on the right, can grow even older – and scientists only have a vague clue how old they can really get. (Image credits: Filippo Fratini (left) and NOAA Ocean Exploration & Research (right), CC BY-SA 4.0)

Patience pays off

But what if we had a warning system for bad conditions… something that told us to just curl up under a blanket and wait for better weather? Nature thought about this too, which is why the life histories of many animals actually include a resting stage. Some examples are sponges, flatworms, rotifers, tardigrades, tunicates and unsurprisingly also cnidarians. This resting stage is often accompanied by a transformation of the individual to a simpler morphology, often a single small tissue clump that can last for a long time until the environmental conditions allow the shift back to full-life.

These mechanisms allow organisms to withstand changing environmental conditions and some can grow very old because of this. This extreme longevity is possible in some animals because they are able to generate new cells replacing the worn-out ones and prevent senescence, the gradual decline of healthy cells. But all of the mentioned animals so far are following that same developmental path, so even if they can survive a long time, they are still heading in one direction – closer to death.

Read More: From Tiny Polyps to the Origin of Stem Cell Research

A (Theoretically) Immortal Jellyfish 

One that steps out of this line is the highly-popular “immortal jellyfish”, a species whose nickname made the headlines in many newspapers. In the 90s Stefano Piraino and his colleagues reared the species Turritopsis dohrnii in their laboratories and what they found was astonishing: the medusa can reverse its life-cycle even from a sexually mature individual and is able to develop back to a polyp colony. This colony is able to asexually propagate as well as producing sexual mature medusae again. 

Transforming back and leaving its predetermined path is a unique feature and was not known to occur in any animal before Piraino’s study. But the underlying process was already known and is called transdifferentiation – the change from an existent, already differentiated cell into another cell type. Simply put, cells that make up for example the gelatinous substance of the jelly of Turritopsis change to another type of cell and get re-organized in a benthic cyst stage, reforming polyp tissue which restarts the life cycle. 

A similar study by Yui Matsumoto and her team in 2019 showed that in each of the mentioned stages of Turritopsis different genes are active – suggested to be associated with the “purpose” of the respective stage. During the polyp and medusa stage, the genes that help form the species’ general shape become very active. The cyst in comparison shows higher activity in genes that are important for repairing the DNA or maintaining the telomeres (these are the ends of the chromosomes which are thought to get shorter with age and work as sort of protective caps). It seems as though these genes prompt the organism to rejuvenate during the cyst stage. 

“All stages of the medusa Turritopsis nutricula, from newly liberated to fully mature individuals, can transform back into colonial hydroids, either directly or through a resting period, thus escaping death and achieving potential immortality.” – Piraino et al. (1996)

[Note: a taxonomical revision of the genus Turritopsis by Peter Schuchert in 2004 revealed that in fact the scientists deal with the species T. dohrnii]

Already before a similar developmental pattern was examined in other hydrozoans almost a century ago. Herbert Müller in 1913 found that Podocoryna carnea could reverse to a polyp stage from a developing medusa. He reared Podocoryna and either stressed or removed developing medusa before they reached maturity. Some of the immature medusae died but others developed back to what he called “Mutterkugeln” – tissue which resembles the larva (nowadays known as the cyst stage). This tissue clump always attached to a nearby surface, yet Müller never saw them growing to full colonies again. It was later confirmed that Podocoryna indeed can grow back to entire polyp colonies and can give rise to new medusae given the right conditions.

When the environment isn’t supportive for producing sexually reproducing medusa, some species won’t waste energy with developing medusa that might not survive long. In 1963 Bernhard Werner showed for example that the species Sarsia tubulosa has a specific temperature limit at which the developing buds revert back to a tissue clump. Under 8°C Sarsia realizes the temperature is too cold, and will start over again, but above 8°C they will develop to healthy medusae. It’s a sort of evolutionary back-up: develop back to a polyp stage, and try the next time the environmental conditions are better.

So what about Turritopsis that makes it so special? Are individuals of the genera Podocoryna, Sarsia and other hydroids or cnidarians not similar? Turritopsis is the only known hydrozoan that can reverse its development in any of its life-stages – even the mature medusa can revert back to a polyp. Before, scientists assumed that if an animal reaches sexual maturity, it’s too late for it to revert back to a younger form. But when faced with developmental stress, such as unfavorable environmental conditions, illness or even signs of aging, Turritopsis can and will reverse its life-cycle back to a cyst and starting the entire cycle all over again.

On the left Sarsia tubulosa polyp with medusa buds, and the medusa on the right (Image credits: L. Martell (left) and E. Degtyareva (right), NorHydro, CC BY 4.0)

Immortality and its Ecological Consequences

Something to consider when thinking about immortality is the high potency of endless growth in population sizes. If we assume that some hydrozoans are close to immortality and thus can grow and propagate in high numbers when the ideal environmental conditions occur, what does this mean for their ecological impact? Maria-Pia Miglietta and her team in 2009 investigated this issue and found that Turritopsis is present in many localities worldwide and that the sampled specimens are genetically very similar. Due to the species high potential for surviving under unfavorable conditions, it might have an advantage in propagating itself and also adapting to many different habitats – and it might even invade new localities without being noticed. 

But still, this developmental feature doesn’t save these organisms from being eaten or getting sick – since external factors are still a caveat when it comes to potential immortality. So we probably don’t need to worry too much about a future ocean with only one species of jelly and polyps. 

Also, Turritopsis might not be the only star on the stage: Doris De Vito and colleagues in 2006 discovered a similar pattern and the potential for reverse development in Laodicea undulata. To this day there are only two species of cnidarians known to undergo full reverse development from a mature medusa but this doesn’t mean that it can’t happen in other animals – maybe so far no one has observed it. We still need much more insights into hydroids life-cycles to improve our understanding of their unique life. 

Wonder why aging and senescence evolved in the first place? And how we can use the scientific insights from other animals to study this topic?

Read part two of this article A Story About Mortality: The Evolution of Aging and Death

Lara Beckmann is a master’s student at the University of Stockholm in the program ‘Biodiversity & Systematics’ and is currently working on her thesis project at the University Museum of Bergen focusing on the diversity of hydrozoans. Follow Lara on Twitter here.

Literature and suggested reading

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. doi: 10.2307/1543022

Matsumoto, Y., Piraino, S., & Miglietta, M. P. (2019). Transcriptome characterization of reverse development in Turritopsis dohrnii (Hydrozoa, Cnidaria). G3: Genes, Genomes, Genetics, 9 (12), 4127-4138. doi: 10.1534/g3.119.400487

Hiebert, L. S., Simpson, C., & Tiozzo, S. (2020). Coloniality, clonality, and modularity in animals: The elephant in the room. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. doi: 10.1002/jez.b.22944

Miglietta, M. P., & Lessios, H. A. (2009). A silent invasion. Biological Invasions, 11(4), 825-834. doi: 10.1007/s10530-008-9296-0