Sharks Holding Their Breath Came as 'Complete Surprise'

Hammerhead sharks dive more than half a mile below the ocean's surface to find and catch prey. And they hold their breath while they do it, according to researchers, who say the discovery—the first time the behavior has been witnessed in fish—came as "a complete surprise." They were interested in learning how scalloped hammerhead sharks handle deep-sea temperatures as low as 39 degrees Fahrenheit, compared to surface temperatures of almost 80 degrees, when they equipped six sharks with sensors to monitor dive depth, water and muscle temperature, body orientation, and tail activity, per New Atlas and the New York Times. What they discovered was that the sharks were sealing their gills and holding their breath for an average of 17 minutes.

Muscle temperature dipped slightly as the sharks began their descent but then returned to the temperature maintained at the surface. "Even when the surrounding water was as cold as 39 degrees Fahrenheit, the sharks had body temperatures around 75 degrees during hourlong dives," per the Times. That was strange, as hammerhead sharks, like other fish, are ectothermic, meaning their body temperatures usually match the temperature of the water around them. With help from computer models, researchers determined the sharks were holding their breath to prevent heat from escaping through their gills at the deepest, coldest depths. The theory is supported by video evidence, collected in 2015, showing a shark swimming at depths of more than half a mile with its gills shut, per the Times.

"Although these sharks hold their breath for an average of 17 minutes, they only spend an average of four minutes at the bottom of their dives at extreme depths before quickly returning to warmer, well-oxygenated surface waters where breathing resumes," says shark biologist Mark Royer of the University of Hawaii at Manoa, lead author of a study published Thursday in Science. He notes "some marine mammals, like pilot whales" do this, too, though "we did not expect to see sharks exhibiting similar behavior." Researchers hope the discovery will reveal "potential vulnerabilities associated with changing ocean conditions or future human exploitation of these deep foraging habitats," which can then be addressed, as the scalloped hammerhead shark is a globally endangered species. (More sharks stories.)