To live for several centuries in icy water, in the slow, eternal night of the abyss, is the fate of the Greenland shark, the species that holds the record for longevity among vertebrates. It lives for at least 300, and sometimes 500 hundred years, according to a study published in 2016.

"It's an emotional experience, to put your hand on an animal that has lived for so long," said Eric Ste-Marie, a PhD student in biology at the University of Windsor, Canada. Fascinated scientists are looking into the mechanisms that could explain the exceptional longevity of this giant fish, which can grow up to 5 meters long. Over the past decade, scientific publications have been accumulating, and the mystery is gradually unraveling.

"There are a lot of ideas. One of them, and it's an important one, is that the shark is living at very low temperatures," said John Steffensen, professor of marine biology at the University of Copenhagen, Denmark, who organizes scientific expeditions to study the animal. Between -1.8°C and 7.5°C. Unlike mammals, which maintain their bodies at a constant temperature, sharks' temperature follow that of their environment. A few hundred meters beneath the pack ice, the Greenland shark's body descends to -1.8°C, the freezing point of seawater. So low, in fact, that the shark's metabolism – all the biological and chemical reactions in its cells – slows to a crawl.

"One of the reasons is that they live in the deep, dark ocean," said Holly Shiels, a professor in the Department of Cardiovascular Sciences at the University of Manchester, UK. "They're in a place that's hard to access for humans. And they have very few natural predators other than humans. So they really can live as long as possible." For the past 10 years, she has been studying the muscular metabolism and physiology of the shark's heart. The animal is slow, very slow. "Even when you let it go, it moves away very slowly," said Steffensen, miming the beating of a fin in slow motion.

This slowness also affects its heart. David McKenzie, director of research at the French National Centre for Scientific Research (CNRS), plunged sharks into a pool on the harbor of Disko Island, Greenland, to take electrocardiograms. "Their hearts beat 4 to 6 times a minute at rest," he said.

A theory known as "pace-of-life" links longevity to the "speed" of metabolic functions. A hummingbird has a very fast metabolism and heart rate and lives for around three years, while the Greenland shark, with its metabolism slowed by the cold and its very slow heart, can live for at least three centuries. But, according to Shiels, this theory alone doesn't explain its longevity: "There are other species that live up in the same waters as the Arctic, and have the same heart rate, that have lifespans that are 20 years, 40 years. So there's still something unusual about the shark."

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