From the immortal "Hydra" to the centenarian "strange shark" in the deep sea, these "immortal" legends are amazing!

Author: Duan Yuechu and Huang Yanhong

In the world we know, birth, aging, illness and death are the iron laws of life. However, in the wonderful picture of the biological world, there are some special creatures that seem to be able to break this law. They are like "rebels" in the long river of time, ignoring the constraints of biological time and showing amazing "immortality" characteristics.

Hydra: The humble immortal

Hydra. Credit: Photo agent/Marko Koenig via Getty Images

Hydra hydra, a freshwater cnidarian that is no more than a centimeter long, is a relative of the jellyfish and is not very attractive. Their tiny bodies are like simple, unadorned tubes with a wisp of tentacles at one end. But don't be fooled by their ordinary appearance. Under this small shell, there is a surprising secret hidden - they do not seem to age.

This phenomenon goes so far against scientific expectations and evolutionary theory that some people describe it as biological immortality. Of course, they are not truly immortal, and disease, predators or starvation can kill them, but it is not because of the inevitability of aging at the cellular level.

Once upon a time, biologist Daniel Martinez was determined to refute the hype about the immortality of Hydra hydra. He was a graduate student at Stony Brook University in New York at the time (now a professor at Pomona College) and believed that through careful observation and experimentation, he would be able to find evidence of aging in Hydra hydra. He told reporters: "I think... as a multicellular organism, you can't escape aging."

In the lab, Martinez spent four full years (a long time for such a small creature) carefully observing the signs of decline in the hydra, such as reduced reproductive capacity or increased susceptibility to death, which are signs of aging over time. However, he found nothing. In 1998, he admitted in his research results: "The results do not provide evidence of aging in Hydra... Hydra may have indeed escaped aging and may be immortal." In a subsequent eight-year study, he and his colleagues obtained the same results. He even kept a Hydra alive for 12 years without finding any signs of age during this period. If his hiking plan in life had not interrupted his research, he would have wanted to extend this experiment. He said: "I decided to take the Pacific Crest Trail, so I couldn't continue (maintaining the Hydra) because I wanted to hike for five months."

After a lot of observations, Martinez had to admit: "These things will not die." However, he also reminded everyone: "We have to be careful when using the word "immortal" because in my laboratory, we have been killing them. They are not really immortal like superheroes, they just don't seem to keep track of time."

So how does Hydra Hydra achieve this near-immortality? It all starts with their way of reproduction. Hydra Hydra can reproduce both sexually and asexually, and the buds produced can grow into identical clones. More importantly, they can also survive for a long time as individuals. "They have a lot of stem cells," said Selina Juliano, a molecular and cell biologist at the University of California, Davis. Their basic bodies are almost entirely made up of undifferentiated "plastic" cells that can constantly divide and renew themselves, more easily than human heart cells. As long as part of the body is intact, Hydra Hydra can recover and regenerate from almost any damage. About every 20 days, an ordinary Hydra Hydra will be renewed, made up of cells that are completely different from before (although the genes are exactly the same), like a tiny Ship of Theseus.

However, new questions arise: why are their individual cells so resilient? "Their stem cells are constantly making new cells, which is amazing," Juliano said. Except for a few special cases (such as Henrietta Lux HeLa cells), other animal cell lineages have a limited number of divisions. With each reproduction, cells accumulate damage, shorten their telomeres, and gradually age. Although researchers are constantly exploring, the specific mechanism by which Hydra Hydra cells avoid aging remains a mystery. However, the study found that they do have active telomerase, an enzyme that can regenerate the long protective tails of chromosomes, which may be part of the puzzle. In addition, according to a 2020 study by Juliano's laboratory, Hydra Hydra appears to have the ability to suppress jumping genes. Through further DNA analysis, she hopes to find more clues.

Greenland shark: A long-lived giant shark in a cold ocean

Greenland shark. Image credit: Hemingway 1952/Wikimedia

In contrast to the tiny Hydra, Greenland sharks are gigantic, reaching lengths of up to 24 feet and weighing more than 2,000 pounds. They are the largest species of shark and rule the cold, far northern oceans as top predators. They are also the longest-lived vertebrates known. A 2016 study dating eye lenses collected from 28 female sharks found that the oldest shark was about 400 years old, and possibly even more than 500 years old. What's more, the sharks don't seem to start breeding until they are around 150 years old, which means they have a childhood and adolescence that lasts for up to a century and a half.

There are likely a number of factors that contribute to the longevity of these behemoths. One key factor is their ability to repair their DNA, says Arne Sam, a computer scientist, biologist, and bioinformatician at Ruhr-Universität Bochum in Germany. When Sam and his colleagues studied the genetic code of Greenland sharks, they discovered that these animals have unusually large genomes with many duplicated genes. Some of the copies are genes that encode proteins that repair DNA and suppress cancer. "The idea is simply that if you have more genes involved in this process, it allows you to survive better and have more DNA repair," Sam explains.

Other studies have found that enzyme proteins encoded by shark DNA are uniquely robust, able to maintain consistently high levels of activity for decades. Generally, as animals age, their enzyme activity and overall metabolism deteriorate, but Greenland sharks do not seem to be affected by this pattern.

The ageless jellyfish: a unique miracle of regeneration

Turritopsis dornhill. Image credit:bachware/Wikimedia

Among cnidarians, Coreopsis dorsalis (also known as the immortal jellyfish) also has the magical ability to subvert aging. Unlike the Hydra polyp, which continues to stay in the polyp stage and renew its cells, Coreopsis dorsalis first grows into a mature jellyfish and then magically reorganizes and reverts to its previous juvenile state, a phenomenon that is very similar to an aging reversal phenomenon called "transdifferentiation."

Like the Hydra, maintaining long telomeres seems to be part of this magical cycle, and like the Greenland shark, they are masters of genetic and cellular repair.

However, there are different opinions on the "immortality" of this jellyfish. In Martinez's view, the cell recombination of jellyfish is more similar to asexual reproduction - producing cloned offspring rather than the continued survival of individuals. He explained: "It regenerates into a ball of cells and produces new polyps from there, but the previous medusa has disappeared. This is interesting, but it is a different phenomenon... The degeneration that occurs can be called aging."

Juliano believes that the phenomenon of Hydra and Coreopsis may suggest that many other related species also have their own rules of aging and regeneration. "Maybe this is not uncommon among cnidarians," she said. "Maybe it's just a matter of degree of research, some have been studied, and some have not been explored in depth." Even this phenomenon may extend to other biological branches, such as some planarians (or flatworms) seem to have unlimited regeneration capabilities similar to cnidarians. Although there is currently limited research on longevity of planarians, "people just think they are basically immortal."

Naked mole rats: long-lived, social rodents that live underground

Naked mole rat. Credit: mari Jan Murat/Photo Alliance via Getty Images

Naked mole rats are nearly hairless, underground, social rodents that have a caste-like social structure similar to that of ants and bees. Among their many unique characteristics, naked mole rats are particularly notable for their longevity. Compared to other rodents and small vertebrates in general, they live an astonishing 40 years, 12 years longer than their closely related African porcupines, which live up to about 28 years.

Although they are still far from immortality, naked mole rats, like hydras, show no noticeable decline in health and physiology as they age. This is partly due to their slow metabolism, extremely slow breathing, cells that tolerate lack of oxygen, and low basal body temperatures, making them cold-blooded animals. In addition, naked mole rats have an amazing ability to resist cancer, which is common in elderly rodents and rampant in other rodents, and naked mole rats resist cancer through "early contact inhibition." When cells begin to crowd together and squeeze each other (similar to early tumor formation), a mechanism is triggered to prevent cell growth. Hyaluronic acid, which triggers this cellular anti-cancer cascade, is a common ingredient in beauty and anti-aging products, but its function in these products is different from that in naked mole rat cells.

Cayman turtle. Image from: Sediment photos

Among turtles and tortoises, some tortoises have extremely long lifespans and can become "stars" at around 200 years old. In fact, many turtles and tortoises do not show obvious signs of aging, especially in captivity. A 2022 study in Science magazine showed that three-quarters of the 52 species of Testudinata surveyed had basically no signs of aging, that is, the risk of death did not increase year by year. But this only applies to turtles and tortoises in zoos and aquariums. The mortality rate in the wild is much higher than in zoos. This is similar to the trend of changes in human life expectancy over the past century. Improving environmental conditions can make organisms live longer and healthier. However, the mechanism by which turtles and tortoises reduce or eliminate aging in captivity remains to be solved, and it is clear that the environment and aging interact.

Amazing creatures such as the Hydra hydra, Greenland shark, immortal jellyfish and naked mole rat challenge our understanding of aging and life limits in unique ways. They are the shining pearls of the biological world, guiding scientists to explore the mysteries of life. In the future, we may be able to find key clues to unlock the mystery of aging from them, bringing hope for human health and life extension.

Scientists are still studying these creatures. For the Hydra hydra, researchers hope to solve the mystery of cell immortality through in-depth DNA analysis; for the Greenland shark, exploring the gene control enzyme protein stability and DNA repair mechanism will help clarify the source of its longevity; for the immortal jellyfish, clarifying the regulatory mechanism of genes and cells during transdifferentiation can help understand the phenomenon of regeneration; for the naked mole rat, studying its physiological structure and gene expression may reveal the secret of longevity.

These seemingly "ageless" creatures span different ecological environments and categories, from microscopic to macroscopic, from freshwater to marine, from underground to deep sea. Their existence reminds us that the diversity and complexity of life far exceeds imagination. We have just started on the road of life science exploration, and there are still countless mysteries to be solved. Are there other "ageless" creatures in the wider biological world? What is their anti-aging mechanism? With the advancement of science and technology and the deepening of research, perhaps there will be a new "immortality" miracle in some corner, which may rewrite our understanding of life and expand the boundaries of life science.

We can imagine that there may be creatures in some mysterious corner of the earth that resist the erosion of time in a special way, such as unknown species in the deep sea or tropical rainforests, which may open the door to a new field of life cognition for us. Moreover, the research results of these "immortal" creatures have a huge potential impact on humans. If we can understand their anti-aging mechanisms, we may be able to develop drugs or treatments to delay human aging, such as drawing on the self-renewal ability of Hydra hydra stem cells or the DNA repair mechanism of Greenland sharks to provide new ideas for fighting aging-related diseases.

From another perspective, the existence of these creatures makes us re-examine the value and meaning of life. If life can break free from the constraints of aging, how will social, cultural and ethical concepts change? This is not only a question for scientists to think about, but also a challenge facing the entire society.

In future research, interdisciplinary cooperation is crucial. The integration of multidisciplinary knowledge and technology such as biology, genetics, chemistry, and physics can provide a more comprehensive perspective for a deeper understanding of these organisms. For example, advanced imaging technology in physics can clearly observe changes at the cellular level, chemical methods can analyze special molecules and enzymes in the body, and mathematics and computer science can build models to predict and understand complex life processes.

The "ageless" characteristics of these magical creatures are like a beacon, illuminating the path to explore the mysteries of life, inspiring scientists to pursue the true meaning of life and solve the puzzles of aging and immortality. As witnesses of the exploration process, we look forward to new discoveries bringing more possibilities and hope to the future of mankind. The research on these "ageless" creatures has immeasurable value, whether it is to improve human health or to satisfy curiosity about the essence of life.

References:

Are any animals truly 'immortal'? These creatures defy biological time. | Popular Science