Sharks holding their breath came as a ‘complete surprise’

Hammerhead sharks dive more than half a mile below the surface of the ocean to find and capture prey. They hold their breath as they do so, according to the researchers, who say the discovery — the first time the behavior has been seen in fish — came as “Complete surprise. “They were interested in seeing how scalloped hammerhead sharks cope with temperatures as low as 39 degrees Fahrenheit, compared to surface temperatures of more than 100 degrees, when they fitted six sharks with sensors to monitor dive depth, water and muscle temperature, direction of body, and tail activity, per New Atlas and the The New York Times. What they discovered was that the sharks were closing their gills and holding their breath for an average of 17 minutes.

The muscle temperature decreased slightly when the sharks began to descend but then returned to the temperature that was maintained at the surface. “Even when the surrounding water was as cold as 39 degrees Fahrenheit, the sharks’ body temperatures were around 75 degrees during an hour-long dive,” according to times. That was strange, because hammerhead sharks, like other fish, are ectothermic, which means their body temperatures usually match the temperature of the water around them. With the help of computer models, researchers determined that sharks were holding their breath to prevent heat from escaping through their gills at the deepest, coldest depths. This theory is supported by video evidence, collected in 2015, that shows a shark swimming at depths of more than half a mile with its gills closed, according to times.

The shark biologist says: “Although these sharks hold their breath for an average of 17 minutes, they spend an average of only four minutes bottom diving at extreme depths before quickly returning to the warmer, more oxygenated surface waters where they are. Breathing resumes,” Mark Royer of the University of Hawaii at Mānoa, lead author of a study published Thursday in Sciences. He noted that “some marine mammals, such as pilot whales” also do this, although “we did not expect to see sharks exhibit similar behavior.” The researchers hope the discovery will uncover “potential vulnerabilities associated with changing ocean conditions or future human exploitation of these deep-sea habitats,” which can then be addressed, since the scalloped hammerhead is a globally endangered species. (Read more shark stories.)