In the shimmering waters of lakes and ponds, a creature exists that defies one of the most fundamental rules of biology: aging. The hydra (Hydra vulgaris), a tiny freshwater organism no bigger than a grain of rice, seems to live forever.

Hydras are essentially biologically immortal, which means they don’t exhibit the typical signs of aging. They can regenerate lost body parts, heal injuries, and reproduce without the usual decline associated with old age.

Now, zooming in on this fascinating freshwater creature, scientists may have found answers to one of biology’s strangest puzzles: contagious cancer. This discovery could reshape our understanding of how tumors evolve and spread.

What Makes Hydras Immortal?

The secret to the hydra’s eternal youth lies in its incredible ability to regenerate. Unlike most organisms, hydras maintain a continuous cycle of renewal and scientists believe this regeneration is due to the presence of stem cells that stay active throughout the hydra’s life. Every 20 days, the whole organism renews itself. In humans, most stem cells are only active while we are developing as embryos.

In fact, in most organisms, stem cells slow down or die off over time. This cellular exhaustion leads to aging and, eventually, death. But in hydras, the stem cells keep going, constantly replacing worn-out tissue. It’s as if the hydra’s body is in a state of perpetual youth, never growing old, never succumbing to the passage of time.

“They’re thought to never die unless you try to kill them or starve them,” says Ali Mortazavi, a developmental biologist at the University of California, Irvine.

And this regenerative power isn’t limited to simple tissue repair. Hydras can even regrow an entire head, a feat that has left researchers wondering exactly how it happens at the molecular level.

Hydras: Immortal but Cancer-Prone

Despite their famous ability to seemingly live forever and without aging, scientists have recently found that they are nevertheless prone to cancer. New findings published in Proceedings of the Royal Society B have shown that these tumors can be transmitted from one generation of hydras to the next.

Transmissible cancer sounds like the stuff of nightmares, but fortunately it is exceedingly rare — though not unheard of. For instance, tumors capable of spreading like infections have ravaged populations of Tasmanian devils, dogs, and certain marine animals. But how does cancer make the leap from one individual to another? And why does this happen so infrequently?

In their new study, the international team of researchers fed a group of hydras two-thirds more food than usual. Previously, scientists had noticed that wild hydras that were overfed in captivity developed tumors. And this is what happened in the new study too.

But what was remarkable was that the tumors were passed on to their clonal offspring, giving scientists a unique opportunity to observe cancer transmission from the very beginning. About one-quarter of the hydras that budded from the tumor-containing parent went on to develop tumors themselves.

A New Model for Studying Cancer

Hydras are not an obvious choice for cancer research. Yet, as Michael Metzger, a biologist from the Pacific Northwest Research Institute who was not involved in this research, explains, they offer an invaluable glimpse into the biology of cancer. “You can really learn a lot about fundamental aspects of cancer biology from them,” he told Science.

The study, led by evolutionary ecologist Sophie Tissot of the French National Centre for Scientific Research (CNRS), found that the tumors were four times more likely to appear in the descendants of hydras with cancer than in descendants of tumor-free animals. And this was even though all the animals were genetically identical. This suggests that the cancer was being transmitted, not simply arising spontaneously.

“This work makes the first contribution to understanding the conditions of transmissible cancer emergence,” the authors wrote.

In fact, the researchers were surprised by how easily the tumors were passed on. This easy transmission challenges a commonly held assumption that such cancers require a “perfector storm” of factors. But, if that’s the case, why are transmissible cancers so rare?

Factors Influencing Hydra Cancer

There are likely other factors are likely at play. The researchers note that diseased individuals are quickly removed from the population by predators, cutting the chain of transmission. The hydra tumors were also “fragile”. Reducing food intake stopped the transmissibility of cancers by the fourth generation.

The study also highlights how changes in a hydra’s environment or life cycle could influence the transmission of cancer. Researchers found that hydras with transmissible tumors had altered life history traits, such as reproducing more rapidly before the tumors fully developed. This adaptation may help them offset the cost of the tumors.

Metzger believes studying hydras could also provide insight into human cancers. While metastasis—the spread of cancer cells from one part of the body to another—is not the same as tumor transmission between individuals, it may share underlying mechanisms.

“It shows the breadth of what cancer can look like,” Metzger said.

Though transmissible cancers remain rare, this research emphasizes the importance of understanding the environmental factors that may encourage their spread. As human activities continue to disrupt ecosystems, the possibility of facilitating such diseases cannot be ignored.