The Chilean devil ray is one heck of a swimmer—the fish make surprisingly fast and deep dives into the Atlantic Ocean, a new study says.
When scientists first put satellite tags on, they expected the devil rays to stay close to warm surface waters, where they’re frequently seen. But their research, published July 1 in Nature Communications, shows that the rays frequently descend more than 5,900 feet (1,800 meters) into a part of the ocean that few other species are able to reach.
The animals also race down as fast as 12 to 13 miles (19 to 20 kilometers) an hour. In comparison, sperm whales and beaked whales typically dive at about 2 to 4 miles (3 to 6 kilometers) an hour; Atlantic bluefin tuna and yellowfin tuna dive at about 9 to 11 miles (14 to 18 kilometers) an hour.
Not only are the fish faster and deeper divers than most other fish, they also travel farther than thought. The study revealed that Chilean devil rays migrate more than about 2,400 miles (3,800 kilometers) over the course of about six months—that’s well beyond the distances traveled by other species of devil rays previously studied in the Pacific Ocean or Mediterranean Sea. (See National Geographic’s pictures of rays.)
The depths and speeds of the dives are “shocking,” says lead author Simon Thorrold—so much so that at first, co-author Camrin Braun, a PhD student who deciphered the data sets, thought he’d made a mistake. But as recalculations produced the same results, they team knew they’d discovered something rare.
“There’s no way I thought they were going that fast,” said Thorrold, senior scientist in the biology department at the Woods Hole Oceanographic Institution.
Whale sharks, northern elephant seals, and Cuvier’s beaked whales are among the only species known to dive as deep as devil rays—most large predators can’t handle the frigid temperatures, low oxygen, and high pressure found below depths of 3,200 feet (1,000 meters). (See pictures of deep-sea creatures.)
Those that can are specially equipped to keep their bodies warm as they dive into waters where temperatures are lower than 39 degrees Fahrenheit (4 degrees Celsius).
No one understood why, though, since devil rays were thought to live in warm, shallow waters. For the new study, between 2011 and 2012, scientists tagged 15 devil rays off the Azores (map), a Portuguese island chain in the North Atlantic where the animals regularly congregate. The tags were programmed to automatically drop off the animals after six months, at which point they float to the surface and send data to a satellite.
The discovery of the rays’ deep dives offers an explanation—and that, in turn, explains why they’re swimming so fast. (Also see “Mystery Solved: How Do Dolphins Swim So Fast?“)
“They use [their] muscles to generate as much heat as possible on the way down,” said Thorrold. Body heat is crucial for brain and eye function.
This discovery is “brilliant,” Les Kaufman, a marine biologist at Boston University who was not affiliated with the study, said in an email.
He said the study is another “striking example” of the apparent link between speed and generating heat.
The fish also warm themselves by soaking up sunlight near the surface. Most of the rays prepare for and recover from dives by spending at least an hour near the surface before and after daytime dives, which are most common. Rays that dive at night don’t spend time near the surface before or afterward.
Anatomy of a Dive
During the dives, which typically last 60 to 90 minutes, the rays rapidly descend, then start to slow down and swim horizontally as they reach the desired depth.
This likely means they’re foraging, hunting for fish and squid—though scientists haven’t actually observed them feeding.
“It’s hard to imagine what other behavior they’re doing down there,” said Thorrold.
Added study co-author Pedro Afonso, a marine biologist at the Institute of Marine Research in the Azores, “We don’t know why they depend on these deepwater layers in their everyday life.”
“But maybe these are layers that we need to protect in order to protect the species.” (Also see “New Study: 1 in 4 Sharks and Rays Threatened With Extinction.”)
Curiously, the scientists recorded three devil ray dives that lasted about 11 hours. “If the rays are down for that long, they’re going to get cold, so they’re going to lose rapid functioning of the brain, and vision will suffer,” Thorrold said.
Basking sharks are the only other large ocean predator known to stay in such cold waters for extended periods. Thorrold and team aren’t sure what the rays are doing during those long-lasting dives, but it’s likely something other than foraging.
To learn more about these superb divers, the team will have to use different types of tags, including acoustic tags that can be left on for several years. Future studies will also give greater insight into the importance of the deep ocean and its inhabitants.
“The discovery that they dive and forage so deep and so regularly,” said Boston University’s Kaufman, ”reinforces a slowly growing sense that open ocean creatures utilize a great deal more of the ocean’s volume than we had realized.”