North Atlantic OceanA tornado of silver horse mackerel and orange boarfish swirls around the submersible as it sits on the seafloor, 770 feet below the surface off the Azores archipelago.
Inside, marine scientists Jorge Fontes and Melissa Cristina Márquez, as well as submersible pilot Lee Frey, are waiting patiently, eating chocolate and making small talk. Through this torrent of fish, they’re hoping to catch a glimpse of the deep sea’s largest predator—the bluntnose sixgill shark.
Then, to their utter delight, a massive figure emerges from the darkness and approaches the submersible: an 18-foot female, the first sighting of the expedition.
Sixgill sharks, three species of which live worldwide, are unlike any other. As their name implies, they have six gills instead of the usual five. The ancient creatures—which sport olive-brown skin, tiny dorsal fins, and haunting emerald eyes—also haven’t changed much since they shared the planet with T-rex and other dinosaurs.
Intriguing as they are, scientists still know very little about these apex predators of the deep sea. The International Union for Conservation of Nature lists the bluntnose sixgill as near-threatened, though data is extremely scarce. This is largely due to the fact that studying these sharks is not easy: Scientists can catch them with nets and hooks and pull them up to the surface, but the experience can be traumatic for the animals.
But in 2019 scientists from Florida State University, the Florida Museum of Natural History, the Cape Eleuthera Institute, and OceanX—an ocean exploration and media initiative established by Ray Dalio and his son Mark Dalio—found a better way to study these sharks: by tagging them where they live, hundreds of feet below the surface. OceanX provided a submersible outfitted with a pair of spear guns modified to inject a satellite tag into the shark’s thick skin. After much trial and error, the team attached a tag onto a bluntnose sixgill in the Bahamas.
The success of the Bahamas expedition energized sixgill research. On June 3 an OceanX team on their research vessel, OceanXplorer, embarked on an expedition off the Azores in the North Atlantic Ocean, working with local scientists to attach satellite and camera tags to the large population of bluntnose sixgills that live among the underwater canyons and seamounts surrounding the islands. (Read about six sharks you’ve likely never heard of.)
“We really are just scratching the surface with what we actually know about sixgill sharks,” says Márquez, a marine biologist at Curtin University in Western Australia. “So attaching [more tags in the Azores] is going to shed some light on these animals that really rule that part of our oceans.”
The sixgill research will be featured in the six-part National Geographic series OceanXplorers, executive produced by James Cameron, BBC Studios' Natural History Unit, and OceanX.
And so far, the expedition has spotted many more sixgills than expected, Fontes says.
“Previously, when we were fishing these animals for deploying tags, we would essentially have one or two animals per night, but here we've been having 10, 11 encounters per night,” he says. “This suggests this particular ecosystem is healthy.”
The right tools for the job
To track sixgills, scientists onboard OceanXplorer—a 286-foot-long former oil drilling support vessel—have at their disposal two submersibles, a remotely operated vehicle, and a research vessel outfitted with equipment that can scan the water column and map the seabed. (Get an intimate look inside OceanXplorer.)
Their goal during the week-long expedition is to outfit as many bluntnose sixgill sharks as they can with two types of tags. One is a satellite tag with a nine-month life span that will document the shark’s vertical movements, and the other is a camera tag that can not only film the shark over an eight-to-12-hour period but also track its location, speed, depth, and surrounding temperature.
Pacific angelshark. Santa Catalina Island, California
Both tags will be attached to the tips of spear guns mounted on the front of the science submersible. Because sixgill sharks have very thick skin, attaching tags to them in this way is minimally invasive.
Perhaps one of the most mystifying behaviors of sixgills is their visits to shallow waters, despite the fact they spend most of their time at depths of up to 4,500 feet.
Off the Azores, just over 800 miles west of Portugal, sixgills likely come to the shallows to feed, but they prefer to spend their time in deep water, where temperatures are cooler, says Fontes, at the University of Azores. (See nine photos of deep-sea creatures.)
With the Atlantic Ocean projected to warm by at least 2.7 degrees Fahrenheit by 2050, it’s possible that sixgills won’t be able to tolerate water temperatures in the shallows, hindering their ability to feed, says Fontes, who’s studied sharks for over 15 years.
“It will be very interesting to analyze the data that we get from these tags, because we’ll be able to compare it with data from colleagues that we are collaborating with from other parts of the world,” Fontes says.
Doing so “will potentially allow us to understand what impacts climate change and ocean warming might have on the distribution of these animals, and eventually their prey.”
A sea full of sharks
So far, the OceanX Azores team is thrilled with the number of sharks they’ve encountered. The fact that sixgills thrive in the Azores is no coincidence, says Pedro Afonso, who also studies sharks at the University of Azores.
Fishing in the region has always been small-scale, and the European Union banned deep-sea shark fishing in 2012, he says. “That’s why you can put a sub down here and see a dozen sharks.”
David Ebert, director of the Pacific Shark Research Center at Moss Landing Marine Laboratories in California, says he’s looking forward to seeing how the movement patterns of Azores sixgills compares with those of sixgills in other parts of the world.
In recent years, scientists have collected data on sixgill sharks in Hawaii, the Bahamas, and the Gulf of Mexico, which has enabled them to compare the vertical distributions of these sharks in different latitudes.
“Tagging a single shark is not really going to tell you much, except for what that one particular shark does over a period of time,” says Ebert, who is not involved in the OceanX effort.
“However, the cumulation of a number of tagged sharks can tell you a lot about their movements and behavior.” (Take the quiz: How much do you know about the deep sea?)
Back in the submersible, Fontes and Márquez are getting ready to return to the surface.
They saw seven sixgill sharks, two kitefin sharks, and two tope sharks over the course of six hours—an impressive feat, even for scientists with a submarine. They hadn’t managed to tag any sharks on that dive, but they had many more nights ahead to try again.
“It was a bit surreal,” says Márquez, “getting to sit back and see the life down there behaving naturally. It was such a privilege.”