Most dominant shark of the deep sea tagged at depth for the first time

Dean Grubbs thinks great white sharks are boring.

The veteran shark scientist, who has researched different shark species for 30 years, is vastly more intrigued by the little-seen dominant predator of the deep, dark, tropical and temperate oceans: the sixgill shark (most sharks have five gills).

"These things are way cooler than any white shark," said Grubbs, an associate director of research at Florida State University's Coastal and Marine Laboratory.

Sixgills are ancient beasts of the dark ocean, often inhabiting waters some 700 to 3,200 feet (200 to 1,000 meters) below the sea, whereas white sharks are a dominant species found near the surface, sometimes to beachgoers' dismay. "[Sixgills] are the biggest, dominant predator of these depths," Grubbs said. The sharks often grow to 16 feet in length, but can become even larger.

And after multiple failed attempts at tagging a bluntnose sixgill shark with a GPS tracker deep under the water — to improve the understanding of how these elusive sharks live — Florida State University's Grubbs along with a team of researchers from the Florida Museum of Natural History, the Cape Eleuthera Institute in the Bahamas, and deep sea explorers OceanX, have successfully tagged a sixgill shark in its natural habitat at some 1,730 feet below the surface.

The late June 2019 mission in The Bahamas — funded by a collaboration of OceanX, the Moore Bahamas Foundation, and the Bloomberg Philanthropies' Vibrant Oceans Initiative (collectively called "One Big Wave") — can be seen in the video below, released Thursday morning.

While Grubbs was on deck of OceanX's 184-foot exploration vessel the Alucia, shark scientist Gavin Naylor descended in a yellow submersible and found a safe spot to tag the sixgill. It was a fortunate shot.

"It was the last dive on the last day," said Brendan Talwar, a research associate at Cape Eleuthera Institute who helped build the shark tagging device.

The 11-minute video is a quality introduction to both the mission and the elusive sharks, which have been roaming the seas since at least 185 million years ago, when dinosaurs dominated the planet. ("[Sixgills] haven't changed in millions of years," noted Talwar). Here are some highlights from the footage:

• 5:50: a massive sixgill shark, perhaps 16 feet long (5 meters), visits the OceanX submersible

• 7:15: Gaze into the cryptic green eye of a sixgill shark

• 8:30: Naylor, ever coolly, tags a sixgill

Grubbs, though, has tagged dozens of sixgill sharks in the past. Just not in the sixgills' deep realm. Instead, tagging sixgills has inevitably required catching the great creatures on a line and temporarily hauling the sharks up to an alien place — the surface world.

Tagging a sixgill from a deep sea submersible is certainly trying and expensive, but it's significantly better for the animal.

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"We don’t have to expose sixgills to any trauma associated with bringing them to the surface," said Grubbs, noting the warmth, light, hook, and out-of-water anguish the sharks experience.

The GPS tag is designed to pop off the shark after three months, which means the tag should have floated to the surface and pinged a satellite about two weeks ago. The tag hasn't yet pinged. But sometimes the tags can get caught in drifting vegetation, or it might take longer to ping for technical reasons. Talwar has been checking each morning (and often afternoons, as well as just before we spoke).

"They are tough."

Previously, Grubbs' tagging research proved that if a fishing vessel accidentally catches a sixgill and brings the animal to the surface, the sharks wouldn't necessarily die, as was previously thought. Instead, the sharks can be returned to the water where they'll likely swim back down to the dark depths, and survive. "They are tough," said Grubbs.

They're also eerie. Like many deep sea creatures, sixgills creep slowly through the water. In the OceanX footage, the sharks moved placidly around the submersible. That's because food is scarce in the deeps, so moving slowly is a wise adaption to conserve energy, explained Grubbs.

"If you’re living at those deep, cold depths, everything is slow," he said.

And then there's the sixgills' jaw. They are profoundly flexible, allowing the sixgills' serrated teeth and wide mouth to bend across large prey, like a whale, and start sawing through flesh. "They'll carve out chunks," said Grubbs.

The cryptic green eye of a sixgill shark. Credit: oCEANX / ONE BIG WAVE Launching the sub at night. Credit: OCEANX / ONE BIG WAVE

Previous tagging expeditions have already revealed compelling insight into the life of a sixgill. During the day, sixgills stay at colder depths of around 41 degrees Fahrenheit (5 Celsius). But at night, the sharks follow prey upwards, migrating to where temperatures hit around 60 F (16 C), said Grubbs.

That's how Grubbs and company sleuthed out sixgill sharks on this recent mission. In the darkness, the researchers parked the submersible atop a slope and waited for the sharks to swim up. To attract the creatures' interest, the scientists tied fish to a pole that extended out from the submarine.

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While there's much to learn about sixgills, Talwar emphasized there's still much to learn about all shark species, even some of the most ubiquitous sharks, like silky sharks.

"We don't even know enough to effectively manage the most commonly seen sharks that are caught at the surface," said Talwar.

And there are some 500 species of sharks on the planet.

But few are as little-known as the great beasts of the deep, the sixgills, who have sniffed out dead prey on the ocean floors since long before the T. rex even evolved.

"We still know almost nothing," said Talwar.