Why some hammerhead sharks seem to ‘hold their breath’ during dives

Even fish typically maintain their breath in chilly, darkish, deep water.

Scalloped hammerhead sharks dwelling close to Hawaii spend their days basking in heat floor waters. However at night time, these fish hunt for squid and different prey within the chilly ocean depths a whole bunch of meters under the floor. The sharks could maintain on to physique warmth within the frigid waters by suppressing the use of their gills while diving, basically “holding their breath” for round an hour at a time, researchers report within the Might 12 Science.

Whales and different deep-diving mammals are known to hold their breath (SN: 9/23/20). However that is the primary time the conduct has been noticed in diving fish, says Mark Royer, a shark physiology and conduct researcher on the College of Hawaii at Manoa in Honolulu.

Sharks and different fish are ectotherms, which means that their physique temperature is essentially managed by the heat of the water round them. Fish lose and achieve a number of physique warmth whereas respiratory by way of their gills, which snag oxygen from water passing by way of the organ.

“Gills are like large radiators strapped to your head,” Royer says, explaining that they leak warmth. Due to this, a number of shark species within the tropics have a tendency to stay to roughly the primary 100 meters of sun-heated water close to the ocean floor, the place temperatures hover round 26° Celsius. However tags connected to scalloped hammerhead sharks (Sphyrna lewini) — a species present in coastal waters all around the tropics — revealed that these sharks take nightly, hour-long dives as much as 1,000 meters under the floor.

At these depths, water temperatures can get as little as 5° C — far too chilly for a tropical shark. To learn the way the sharks endured such frigid temperatures, Royer and his colleagues connected specifically designed devices to the backs of sharks that had gathered in a shallow bay off Oahu to mate.

For the subsequent 23 days, these sensors tracked how the sharks moved, how deep they swam, and the way their inside temperature modified. “It was sort of like attaching a Fitbit to a shark,” says Royer. “It allowed me to get exact particulars on what the shark was doing.”

Sharks, the information present, went on V-shaped dives to the depths — plunging a whole bunch of meters earlier than firing straight again up “like a missile,” says Royer. However surprisingly, the physique temperature of diving sharks barely budged for the majority of the dive. It was solely when the sharks slowed their ascent at a depth of round 290 meters, the place the water is somewhat cooler than on the floor, that their physique temperature dropped by a median of two.8 levels C.  

The fish needed to be shutting off their gills for a lot of the dive to carry on to their warmth, the researchers concluded. It was solely when the sharks had returned to a safer depth temperature-wise that they might have reactivated their gills — taking in oxygen for the primary time in round an hour and sucking in chilly water within the course of.

Holding on to their warmth whereas diving may assist sharks transfer shortly within the deep ocean, says Julia Spaet, a shark ecologist on the College of Cambridge. Whereas it’s “completely doable” that these hammerhead sharks do that by suppressing gill exercise, scientists might want to get direct proof utilizing cameras or different means to show that it’s true, she says.

Not less than one video from a deep-sea dive hints that that is the case. The gills of a scalloped hammerhead roaming at a depth of 1,000 meters close to Tanzania seemed to be closed in footage captured just a few years in the past, the researchers word of their paper. This, together with the brand new research’s discover that hammerheads maintain on to their physique warmth, makes Royer “very assured” that sharks are in reality holding their breath. “It simply goes to spotlight how extraordinary this species is,” he says.


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