Hydra, specifically Hydra vulgaris, are unassuming creatures. At their largest, the freshwater cnidarians (relatives of jellyfish) are just about a centimeter long. Their miniscule bodies are simple, unadorned tubes with a plume of tentacles at one end. Yet their simplicity belies a stunning secret: They don’t seem to age. It’s an observation that defies scientific expectation and evolutionary theory, and has led some to describe the polyp species as biologically immortal–as in yes, they die (of disease, predation, starvation), but not out of any cellular inevitability.
Initially, Daniel Martínez set out to disprove the hype. The biologist–now a professor at Pomona College, but then a graduate student at the State University of New York, Stony Brook–believed that with careful observation and experimentation he could catch a hydra in the act of growing older, or senescing. “I thought…there’s no way you can escape aging as a multicellular organism,” he tells Popular Science.
For four years (a long time for such small organisms) Martínez looked closely for hallmarks of decline in hydra he kept in the lab–signs they were less reproductive or more likely to die over time. Yet he didn’t find any. “The results provide no evidence for aging in hydra… Hydra may have indeed escaped senescence and may be potentially immortal,” he conceded in the resulting 1998 study. In follow-up research spanning eight years, Martínez and colleagues found identical results. He’s kept individual hydra alive for 12 years at a time, without noting any markers of age. He would’ve happily done it for longer, too, had life not gotten in the way of his research on immortality. “I decided to do the Pacific Crest Trail, so I couldn’t continue [maintaining the hydra] because I wanted to hike for five months,” he says.
Amid all the observations Martínez did manage to collect, he eventually admitted to himself, “these things are not dying,” he says. The skeptic was convinced–with one caveat: “We have to be careful when we use the word ‘immortal’, because we kill them all the time in my lab,” he says. “They’re not really immortal [as in invincible], they just don’t seem to register time.”
Hydra are a stand-out example of the phenomenon, but they aren’t necessarily unique in their resistance to time. A handful of animals display unexpected longevity and negligible aging. Slowly, science has revealed details of and theories for the varied ways these organisms avoid growing old.
How do Hydra do it?
Hydra reproduce both sexually and asexually, creating buds that grow into identical clones. Yet they don’t just survive on through these duplicates, but also as individuals. Why? “They have a lot of stem cells,” says Celina Juliano, a molecular and cellular biologist at the University of California, Davis. Their basic bodies are almost entirely made up of undifferentiated, “plastic” cells that can continually divide and self-replace much more readily than, say, a human heart cell. Hydra can recover and regenerate from nearly any injury, as long as a piece of their central body is intact. Every 20 days or so, an individual H. vulgaris is renewed, composed of entirely different (though identical) cells from before, like a tiny Theseus’ ship.
But still, that leaves the question of how their individual cells are so resilient. “That their stem cells endlessly make new cells is remarkable,” says Juliano. With few exceptions (e.g. Henrietta Lacks’ HeLa cells), other animal cell lineages have a set number of times they can divide. With each subsequent multiplication, they accrue damage, lose telomere length, and age. How, exactly, hydra cells avoid petering out remains a mystery, though researchers are making strides. They do have active telomerase, the enzyme that allows the long, protective tails of a chromosome to regenerate, says Juliano–so that’s likely one piece of the puzzle. Another seems to be hydra’s ability to keep wily, jumping genes suppressed, according to 2020 research from Juliano’s lab. Through further DNA analysis, she’s hoping to uncover more clues.
Greenland sharks
Counter to hydra, Greenland sharks are massive–measuring up to 24 feet long and weighing more than 2,000 lbs. They’re the largest species of shark, reining over cold, far northern oceans as apex predators. They’re also the longest-lived, known vertebrate species. One 2016 study dated the eye lenses collected from the bodies of 28 female sharks and found that the oldest was about 400 years old, and possibly more than 500 years old. The sharks don’t even seem to enter their reproductive years until around 150–one and a half centuries of shark childhood and adolescence.
There are likely many contributing variables for how these big fish manage to live hundreds of years, but one factor is their ability to repair their DNA, says Arne Sahm, a computational biologist and bioinformatician at the Ruhr-University Bochum in Germany. Sahm and his colleagues have been studying Greenland sharks’ genetic codes. They’ve found that the animals have an exceptionally large genome, with many duplicated genes. Some of these duplicates are genes that code for DNA-repairing and cancer-suppressing proteins. “The idea is pretty simple, if you have more genes involved in this process it allows you to survive better and have more DNA repair,” Sahm tells Popular Science.
Other research has identified that the enzyme proteins encoded by the sharks’ DNA are uniquely robust, maintaining consistently high levels of activity, even over decades. Generally, as animals age, their enzyme activity and overall metabolisms show, but Greenland sharks don’t seem to experience the same degradation.
Immortal jellyfish?
Hydra are not the only seemingly ageless cnidarians. Turritopsis dornhii, otherwise known as the immortal jellyfish, have their own, unique way of subverting senescence. Instead of staying continually in the polyp phase, like hydra, and swapping out their individual cells, T. dornhii grows into a mature medusa and then, miraculously, reorganizes and reverts to its former juvenile self in a phenomenon that looks a whole lot like aging backwards called “transdifferentiation.”
As with hydra, maintaining long telomeres seems to be part of what enables this miraculous, circular cycle. And as with Greenland sharks, these jellies are masters of genetic and cellular repair.
Though not everyone is convinced Turritopsis’ trick amounts to the same sort of marvel as what the hydra manages to do. In Martínez’ view, the jellyfish’s cellular reorganization is much more akin to asexual reproduction–producing a clonal offspring–than it is to continued survival of an individual. “It regenerates into a ball of cells and from there it produces a new polyp, but that medusa from before has disappeared,” he explains. “It’s fascinating, but it’s a different phenomenon… the degeneration that happens, that could be called aging,” he adds.
In Juliano’s view though, what happens in hydra and in Turritopsis could indicate that many other related species also abide by their own rules of senescence and regeneration. “In cnidarians, maybe it’s not exceptional,” she says–maybe it’s just a matter of what has and hasn’t been studied. And perhaps that extends to other clades as well. Some planarians, or flatworms, seem to have a similar, cnidarian-like ability to infinitely regenerate. Limited longevity studies have been done, says Juliano, “but I know people sort of just view them as essentially immortal.”
Naked mole rats
Naked mole rats, the nearly hairless, subterranean, eusocial rodents with a hierarchical structure that resembles ants and bees stand out in many regards. One item on their long list of quirks is their lifespan. Relative to other rodents (and small-bodied vertebrates in general), mole rats live remarkably long lives. They’re known to survive up to 40 years, 12 years longer than their closest rodent competition, the African porcupine which lives up to about 28 years.
Though this is infinity years away from immortality, mole rats, like hydra, approach their later years with no noticeable declines in health or physiology. Part of this is their relatively slow metabolisms. Naked mole rats breathe very slowly, their cells are resistant to oxygen deprivation, and they have such low baseline body temperatures that they’re effectively ectotherms (or coldblooded). Then there are other factors at play.
Mole rats are particularly impressive for their ability to ward off cancer, a disease of old age, which is rampant among other rodents. Research indicates they do this via “early contact inhibition,” says Sahm. When their cells begin to grow dense and bump up against each other–as in early tumor formation–it triggers a mechanism that stops cell growth in its tracks. The compound that sets off the cellular anti-cancer cascade, hyaluronic acid, is a common ingredient in beauty and anti-aging products–though its touted function in serums and creams is much different from what it does within mole rat cells.
Turtles and tortoises
Some tortoises live so long that they manage to become celebrities in their second century. But it’s not just a select few testudines that show a remarkable ability to evade death. Many species of turtle and tortoise display negligible aging, particularly in captivity, according to 2022 research published in Science. Three quarters of the 52 species surveyed showed essentially no senescence. In practice, what this means is that year to year their risk of mortality didn’t increase, says Fernando Colchero, a statistical demographer at the Max Planck Institute for Evolutionary Anthropology in Germany.
However, this trend only held true for animals held in zoos and aquariums. “In the wild, rates are much steeper than in the zoos,” Colchero says. This, he argues, illustrates the same sort of trend that human life expectancy over the past century shows–when you improve the environmental conditions of an organism, it can live a much longer, healthier life. How, exactly, turtles and tortoises reduce or eliminate aging when kept in captivity remains an open question, Colchero notes, but it’s clear there’s an interplay between environment and senescence.
Lobsters (and other shellfish)
Lobsters are yet another incredibly long-lived aquatic animal, with some individuals estimated to be well over 100 years old. In addition to aging very slowly, the crustaceans also never stop growing (the official world record-holder was 44 pounds). They’ll get bigger and bigger until all that growth and molting becomes too much to bear. Constructing and shedding ever-larger exoskeletons eventually becomes too energy-intensive, and they eventually fail to molt and die. Though this isn’t senescence in any traditionally understood, evolutionary sense, where cells degrade over time, it is an increase in mortality risk as a byproduct of advancing maturity.
And among the shelled sea creatures, the lobster isn’t the most impressive. Ocean quahogs, a species of clam, can live more than 500 years. One particular specimen, dubbed Ming (after the Chinese dynasty during which it was born), was found off the coast of Iceland in 2006. Counting growth bands on the shell revealed the mollusk had survived 507 annual cycles, before being killed for research. It is the longest-lived, non-colonial animal ever discovered.