One of the last things biologists expected to find in the balmy Caribbean Sea was an ancient Greenland shark,?????? ???? ?? ????? a creature known for dwelling far off, in the icy Arctic.

Yet researchers, while temporarily catching and tagging tiger sharks off the coast of Belize, caught a Greenland shark (or potentially a Greenland-shark hybrid), a species that lives for centuries in the deep sea.

"We suddenly saw a very slow moving, sluggish creature under the surface of the water," Devanshi Kasana, a biologist and Ph.D. candidate at Florida International University's Predator Ecology and Conservation lab, told Mashable. The observation was recently published in the science journal Marine Biology. At first, the researchers thought it could be a sixgill shark, a dominant and fascinating predator of the deep sea. But they photographed the rarely-seen animal and confirmed it was a Greenland shark.

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"It looked like something that would exist in prehistoric times,"Kasana added.

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Indeed, Greenland sharks belong to a family of sharks that are around 100 million years old, existing when dinosaurs dominated the planet. The sharks spend much of their lives in the dark, thousands of feet underwater, where they grow slowly, move slowly, and age slowly. Down in the deep sea, where nutrients are rare, moving slow to conserve energy is an important adaptation. Greenland sharks are clearly well-adapted for these depths: They live for well over two and a half centuries, and perhaps considerably more. They are the longest-lived vertebrate on Earth.

The Greenland shark, with its stark greenish-blue eye, observed in Belize by marine biologists. Credit: Devanshi Kasana

What's a Greenland shark doing in the Caribbean?

Spotting a Greenland shark near a coral reef off Belize was certainly an unexpected surprise. But it's not unimaginable.

This relatively little-known species is known to thrive in the deep seas in and around the Arctic. They could potentially dwell in other deep ocean regions, too, say biologists. This includes the Caribbean. After setting a line in Belize's protected Glover’s Reef Atoll while monitoring and researching tiger sharks, the biologists returned the next day to find their line had moved a couple of miles away from the coral reef, into water some 2,000 feet deep.

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When they pulled up their scientific catch, they saw the unusual shark. "It looked very, very old," marveled Hector Daniel Martinez, one of the researchers who spotted the shark and a coauthor of the study. "It was in very deep water."

"It looked very, very old."

The slope off the nearby reef plummets down to some 9,500 feet deep. It's a profoundly cold, dark realm, ideal for a Greenland shark.

The deep seas are famously little explored and not well understood. The discovery of this Arctic shark underscores that just because we haven't seen a phenomenon, doesn't mean it isn't occurring. "We know so little about the deep ocean that pretty much anyone can find something new if they were doing something unique down there," Alan Leonardi, the director of the National Oceanic and Atmospheric Administration's Office of Ocean Exploration and Research, told Mashable in 2020.

Finding a Greenland shark in Belize wasn't easy. It required diverse researchers, local fishers, and the Belize government collaborating in a protected area of the ocean. It gave researchers the opportunity to observe something scientifically unprecedented. "This discovery is made possible by scientists working together," Demian Chapman, one of the study's coauthors and director of Sharks and Rays Conservation Research at Mote Marine Laboratory and Aquarium, told Mashable.

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"It was very close to coral," noted Chapman. "You normally think of them as being close to ice."

A looming question is if this particular Greenland shark had traveled to the Caribbean from Arctic seas, or if it had lived in (deep) tropical waters for much of its life. It's unknown. But there's a good chance there's more of them roaming down there, in the dark waters where we can't see.

"I doubt it's the only one," said Chapman.

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