Hammerhead sharks like it warm, but for a good meal they’re willing to get cold. The flat-headed predators dive more than 2,600 feet from tropical surface waters into the ocean’s frigid depths multiple times every night to hunt for fish and squid, tolerating a 68-degree Fahrenheit plunge in temperature to dine.
How do these coldblooded chondrichthyans tolerate these temperatures without turning into frozen fish? A study published Thursday in the journal Science shows how one species, Sphyrna lewini or scalloped hammerhead sharks, stay warm during their nightly dives: They skip the frills and close their gills, essentially holding their breath.
This strategy for regulating a coldblooded fish’s temperature has never been observed before and distinguishes them from high-performance fish (yes, that’s the scientific term) like great white sharks or Atlantic bluefin tuna that use vastly different strategies to tolerate extreme cold.
Mark Royer, a shark biologist at the University of Hawaiʻi at Mānoa, was inspired to investigate the scalloped hammerhead’s secret heating technique after noticing how deep they were diving during a different research project. He attached a package of sensors near the dorsal fins of six hammerheads near Hawaii. The packages were designed to detach from the sharks after several weeks and emitted a satellite signal when they were ready to be scooped out of the sea.
The tags were like shark Fitbits, Dr. Royer said, collecting data like depth and body temperature. They were even sensitive enough to detect each individual flick of the fish’s tail. Dr. Royer and colleagues found that the hammerheads lose a little body heat when they start their descent, but then quickly return to the same temperatures they were at the surface as they swim deeper. Even when the surrounding water was as cold as 39 degrees Fahrenheit, the sharks had body temperatures around 75 degrees during hourlong dives.
Sharks are ectotherms, which means their body temperature is largely determined by the surrounding water temperature. Dr. Royer and his team used a mathematical model to show that the temperature data they collected didn’t make sense unless the sharks were somehow actively conserving body heat. They also measured rates of heat exchange between dead scalloped hammerheads (that had washed up on the beach) and a water bath and found rates similar to those between live deep-diving sharks and ocean water. The key similarity between the two? “No conductive heat loss across the gills,” Dr. Royer said. And the gills are the No. 1 source of heat loss in a fish’s body.
“Gills are essentially giant radiators strapped to the head,” he said.
The conserved body heat and the lack of other physical adaptations that could prevent heat loss convinced Dr. Royer that the fish were “holding their breath,” somehow stopping the flow of water over their gills — and their ability to take in oxygen. The researchers suspect the hammerheads do this by physically closing the gill slits, based on a 2015 observation of a scalloped hammerhead doing so more than 3,000 feet below the surface. Dr. Royer wants to attach video cameras to diving hammerheads next to confirm this hypothesis.
Catherine Macdonald, a marine biologist at the University of Miami who was not involved with the study, agreed with the team’s reasoning, saying that she couldn’t “see a way” the sharks could be breathing normally while maintaining the body temperatures seen in the data.
Dr. Royer is next planning to study the hammerheads’ metabolism to better understand the recovery period that follows the extreme athletic feat they perform each night. He suspects that the hammerheads’ propensity for relatively short periods of high activity may explain why they die so easily when trapped on fishing lines for many hours; it’s like asking an elite sprinter to run a marathon.
“This study invites a lot of additional studies,” Dr. Macdonald said. “I am always delighted by sharks’ capacity to surprise me.”