A Greenland shark, one of the longest-living animals on Earth, was caught near Belize

Researchers tagging tiger sharks off the southern coast of Belize couldn't believe their eyes when they recently reeled in a different kind of fish. It turned out to be a Greenland shark, which is typically found in the Arctic and can live to be over 500 years old.

The team of scientists thought the shark was dead when they finally hauled it to the surface. Unlike the tiger sharks they were after, this particular shark had black, worn-looking skin and pale blue eyes. Devanshi Kasana, a Ph.D. candidate at the Florida International University Predator Ecology and Conservation lab, said the shark looked "really, really old."

"It was just very surprising and confusing," she said. "As soon as it entered our field of vision, we saw a black figure that was getting bigger and bigger. When it came to the surface, none of the crew with all of their combined fishing experience had seen anything like that."

Kasana and colleagues published an article on the capture in the journal Marine Biology in July.

Kasana said the discovery is especially exciting because it suggests that these sharks, which were thought to mainly exist in the Arctic, can be found in the tropics as well.

Greenland sharks can likely live to be more than 500 years old

Scientists have more questions than answers when it comes the Greenland shark. According to the National Oceanic and Atmospheric Administration, these sharks are the longest-living vertebrates on the planet, living possibly more than 500 years. And that's just an estimate, because there's no definite way of determining their age.

The reason they live to be so old may have something to do with their speed of life, which is slow — very, very slow. Greenland sharks grow approximately one-third of an inch per year and can grow to more than 20 feet in length. And researchers believe the sharks don't reach sexual maturity until sometime after the first 100 years of their life.

According to Kasana, some scientists have theorized that the Greenland shark can be found across the globe, if one knows where to look. They love cold water, which is why they're found in the Arctic. However, they've also been discovered as far south as off the coast of Georgia, thousands of feet beneath the ocean surface.

The theory is that the closer the sharks are to the equator, the deeper researchers have to go to find them. And they go deep, over 7,000 feet down, according to NOAA.

The catch was unexpected

Kasana said their catch was strictly accidental. On April 22, she and her team were working with members of the Belizean shark fishing community and the Belize Fisheries Department to set lines along Glovers Reef, located about 30 miles off the coast. The waters of the reef can be as shallow as 25 feet, but steep drop-offs can reach more than 2,000 feet down.

"It slopes suddenly and the depth goes really deep really fast," Kasana said. "We believe the line dragged from a much shallower depth to the drop-off, which is why we ended up catching this individual."

A report of the encounter published on July 15 claims this was the first Greenland shark to be discovered in the Western Caribbean. Knowing how rare the experience was, Kasana said her team discussed buying a lottery ticket should they ever come across another one.

"If we were to catch another individual it would be sheer luck, we don't set our lines in a way that targets Greenland sharks," Kasana said.

When they reeled in the shark, it looked extremely old, she explained. And though they briefly considered tagging it, they didn't want to incidentally hurt or kill the shark in the name of science. Instead, Kasana and her team measured the shark, took notes and a photo, and sent it on its way.

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