Hammerhead sharks ‘hold their breath’ to stay warm during deep dives

(CN) — Technically speaking, scalloped hammerhead sharks hold their breath during deep dives. In fact, they must do so to survive.

Scalloped hammerhead sharks hunt for prey in the ocean depths where water temperature can be below 40 degrees Fahrenheit, but do not have the morphological or vascular adaptations to conserve body heat. In an observational first, researchers found hammerheads maintained an elevated body temperature of up to 68 degrees Fahrenheit above ambient temperatures throughout the deepest portion of each dive, and then only rapidly began to lose heat on their return to the surface.

According to a study published Thursday in Science, they do it by tightly closing their mouths and gill slits to keep from breathing in frigid water. After diving, the reopening of gill slits and convective heat transfer upon the shark's return to warmer surface waters causes a rapid loss of body heat.

In a phone interview, study author Dr. Mark Royer of the University of Hawaii at Manoa said the hammerheads can hold their breath for an average of 17 minutes at a time. Royer and his team learned this thanks to their remote biologgers, which Royer said consisted of several different types of animal tags attached to syntactic foams.

Once attached to the hammerhead’s dorsal fin, Royer said that the biologgers generated high resolution data that could not be transmitted. However, Royer said the biologgers could transmit their location when they popped off the hammerheads and floated to the surface.

Because the hammerheads were out at sea for weeks at a time, Royer said the researchers would not know when the biologgers would pop off, where the hammerheads would be, or what the weather would be like.

“And because around Hawaii there isn’t much landmass for the biologgers to drift onto, they could be caught on a current and drift out of our reach,” said Royer. “And sometimes it would be anywhere from 15 to 20 miles offshore, and on those days we would sometimes have heavy seas with five-to-eight-foot swells that we would have to roll through with 15 knot winds.”

Royer considered it a high-risk, high-reward venture, and future studies could benefit from studying the enzymes of the hammerhead’s muscles.

“By looking at whether or not these sharks have the enzyme levels to do high rates of aerobic or anaerobic activity can tell us, physiologically, how these sharks are able to hold their breaths for long periods of time,” said Royer.

Further research on the hammerheads' behavior may aid conservation of the endangered species, whose hunting areas are jeopardized by what Royer called the shallowing of ocean oxygen zones.

“That’s where in many parts of the ocean — including the eastern Pacific where scalloped hammerheads live — there is a growing oxygen dead zone where the oxygen concentration in the water is decreasing, and those zones are expanding both across the ocean and in depths,” said Royer.

Expansion of oxygen dead zones will force the hammerheads to come closer to the surface before they can open their gills, making an already strenuous activity more dangerous. And deep sea mining and twilight zone fishing, which Royer described as fishing at depths between 600 to 3,000 feet where only the faintest amount of sunlight reaches, could greatly affect the prey the hammerheads hunt for.

“In addition to trying to prevent our overfishing of these sharks, we also need to be on the lookout to make sure that these other human impacts don’t affect their already afflicted populations,” said Royer.

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