What makes these little “water bears” so tough? They repair broken DNA quickly.

To introduce her children to the hidden wonders of the animal kingdom a few years ago, Anne de Sien ventured into her garden in Paris. Dr. De Cian, a molecular biologist, collected pieces of algae, then went back inside, soaked them in water and placed them under a microscope. Her children stared through the lens at strange eight-legged creatures climbing over the moss.

“They were impressed,” Dr. De Cian said.

But she's not done dealing with the little monsters, known as tardigrades. She brought them to her laboratory at the French National Museum of Natural History, where she and her colleagues hit them with gamma rays. The explosions were hundreds of times greater than the radiation needed to kill a human. However, the tardigrades survived and continued their lives as if nothing had happened.

Scientists have long known that tardigrades are strangely resistant to radiation, but only now Dr. De Cien and other researchers have discovered the secrets to their survival. The tardigrade appears to be adept at molecular repair, able to quickly reassemble piles of broken DNA, according to a study published Friday and another earlier this year.

Scientists have been trying to penetrate tardigrades' defenses for centuries. In 1776, Lazzaro Spallanzani, an Italian naturalist, described how animals could become completely dry and then be revived with a spray of water. In subsequent decades, scientists found that tardigrades could withstand crushing pressure, deep freezes, and even flight into outer space.

In 1963, a team of French researchers discovered that tardigrades could withstand massive blasts of X-rays. In recent studies, researchers found that some tardigrade species can withstand a dose of radiation 1,400 times higher than what would be needed to kill a person.

Radiation is deadly because it breaks DNA chains. High-energy rays hitting a DNA molecule can cause direct damage; It can also cause chaos by colliding with another molecule inside the cell. This altered molecule may attack DNA.

Scientists suspect that tardigrades could prevent or reverse this damage. In 2016, researchers at the University of Tokyo discovered He discovered a protein called Dsup, which appears to protect the tardigrade's genes from energy rays and stray molecules. The researchers tested their hypothesis by putting Dsup into human cells and bombarding them with X-rays. Dsup cells were less damaged than cells without the tardigrade protein.

This research sparked Dr. De Cian's interest in tardigrades. She and her colleagues studied the animals she collected in her garden in Paris, along with a species found in England and a third from Antarctica. As they reported In January, gamma rays destroyed the tardigrades' DNA, but failed to kill them.

Courtney Clark Hachtel, a biologist at the University of North Carolina Asheville, and her colleagues independently found that tardigrades I ended up with broken genes. Their study was published on Friday in the journal Current Biology.

These results suggest that Dsup alone does not prevent DNA damage, although it is possible that proteins provide partial protection. It's hard to know for sure because scientists are still figuring out how to conduct experiments on tardigrades. They can't engineer animals without the Dsup gene, for example, to see how they deal with radiation.

“We would like to do this experiment,” said Jean-Paul Concordet, Dr. de Cien's assistant at the museum. “But what we can do with tardigrades is still quite primitive.”

The two new studies reveal another trick for tardigrades: they quickly repair broken DNA.

After tardigrades are exposed to radiation, their cells use hundreds of genes to make a new batch of proteins. Many of these genes are familiar to biologists, because other species — including us — use them to repair damaged DNA.

Our cells are constantly repairing genes. Strands of DNA in a typical human cell break about 40 times a day, and each time, our cells have to repair them.

Tardigrades make these standard repair proteins in astonishing quantities. “I thought this was ridiculous,” Dr. Clark Hachtel recalled when she first measured her levels.

Dr. De Cian and her colleagues also discovered that radiation causes tardigrades to produce a number of proteins not found in other animals. For now, their functions remain mostly a mystery.

The scientists chose a particularly abundant protein to study, called TRD1. When introduced into human cells, it seemed to help the cells resist damage to their DNA. Dr. Concordet speculated that TRD1 might grab onto chromosomes and keep them in their correct shape, even when their strands begin to fray.

Studying proteins like TRD1 will not only reveal the power of tardigrades, but may also lead to new ideas about how to treat medical disorders, Dr. Concordet said. DNA damage plays a role in many types of cancer, for example. “Any tricks they use we might benefit from,” Dr. Concordet said.

Dr. Concordet still finds it strange that tardigrades are so good at surviving radiation. After all, they don't have to survive in nuclear power plants or uranium-lined caves.

“This is one of the big mysteries: Why are these organisms resistant to radiation in the first place?” He said.

Dr. Concordet said that this tardigrade's superpower could just be an unusual coincidence. Dehydration can also break down DNA, so tardigrades may use their armor and repair proteins to combat dehydration.

While a Parisian park might seem like an easy place to live, Dr. Concordet said it could pose a lot of challenges for tardigrades. Even the disappearance of dew every morning could be a disaster.

“We don't know what life is like out there in the moss,” he said.