While swimming in cold, deep waters, scalloped hammerhead sharks have an unusual way of keeping warm: They close their gills and hold their breath. This unusual behavior, documented Thursday in Science, has never been recorded in any other fish.
“This was a complete surprise,” Mark Royer, a researcher with the Hawai‘i Institute of Marine Biology's Shark Research Group, says in a statement. “It was unexpected for sharks to hold their breath to hunt like a diving marine mammal. It is an extraordinary behavior from an incredible animal.”
Hammerheads are ectotherms—or cold-blooded creatures—which means their body temperatures are regulated by their environment. Their gills, which let them breathe by picking up dissolved oxygen in the water, are major points of body heat loss, Royer tells Nature News’ Bianca Nogrady. Because the sharks can’t produce their own heat, they become sluggish and unable to swim if they get too cold. And if a shark stops moving, then water can’t flow through its gills, which can lead to suffocation.
Still, hammerheads manage to dive thousands of feet into the ocean’s cold depths in search of prey. But they don’t use any of the known strategies: Tunas, marlins and mako sharks, for example, have heat exchange systems at their gills, per Nature News. A bluefin tuna can use this system to raise its body temperature as high as 38 degrees Fahrenheit warmer than the water. And whale sharks, which can reach up to 39 feet in length, conserve body heat simply because of their size.
To figure out the hammerhead sharks’ strategy, Royer and his colleagues attached specialized sensors to the creatures’ backs in a shallow bay off Oahu. The sensors tracked each shark’s muscle temperature and body orientation, the surrounding water temperature and how deep the animal was swimming.
“It was kind of like attaching a Fitbit to a shark,” Royer tells NPR’s Ayana Archie and Geoff Brumfiel. “It allowed me to get precise details on what the shark was doing.”
The hammerheads dove repeatedly and rapidly from shallow waters at temperatures of about 79 degrees Fahrenheit down 2,600 feet, to waters as frigid as 41 degrees. They remained at that depth to feed for only about four minutes before swimming up again at a steep angle. But their body temperatures remained constant until about 17 minutes into their ascent, when they slowed their movements and cooled down by an average of 5 degrees Fahrenheit.
“We looked at the body temperature data, [and] right off the bat, we could see that these sharks were doing something very interesting and very different, compared to any other fish,” Royer tells Scientific American’s Meghan Bartels. “It’s obviously not simple thermal inertia that these sharks are relying on to maintain their body temperature on these deep dives. There’s something much more complex.”
Computer modeling suggested they must be closing their gills, preventing cool water that would chill their body from flowing across them, per the statement. Additionally, video evidence taken by researchers in 2015 of a hammerhead in Tanzania at a depth of more than 3,400 feet supports this idea, per the study. In the video, the shark appears to have its gills closed, while other footage of scalloped hammerheads in shallow waters show their gills slits open, write the authors.
While it is “absolutely possible” that the sharks conserve heat by closing their gills, further direct evidence using cameras or other methods is needed to confirm this, Julia Spaet, a shark ecologist at the University of Cambridge in England, tells Science News’ Freda Kreier.
But Marianne Porter, a biologist who studies shark movement at Florida Atlantic University and wasn’t involved in the study, tells National Geographic’s Dina Fine Maron: “It’s a compelling paper—the way they have all these lines of evidence suggesting this might be happening,” she says to the publication. “I’m convinced.”