Sharks ‘hold their breath’ to stay warm in deep waters

HAMMERHEAD sharks “hold their breath” when diving deep underwater in order to stay warm, a study has found.

The fish lives in the shallow, warm waters of the tropics but often dives down hundreds of feet to find prey, such as herring, sardines and mackerel.

However, the hammerhead is a coldblooded predator unable to regulate its own body temperature, making it best suited to waters of about 20C (68F).

This poses a challenge when necessity forces the creature to go hunting in the depths of the ocean, where water temperatures can fall to near-zero.

If it gets too cold, it can suffer from a lack of energy as well as visual impairment and potential cardiac issues that could prove fatal.

Hammerheads also need to be moving constantly as they breathe by extracting oxygen from water as it flows over their gills.

To learn how the sharks balance these biological restrictions with the need to dive into frigid water for food, scientists at the University of Hawaii fitted trackers to some scallop hammerheads in the Pacific and monitored them over several days in 2017.

They found that they slowly descend until they reach a depth of about 300ft and then they enter a near-vertical dive, rapidly accelerating as far as half a mile underwater.

They would then stay at this depth for about four minutes, searching for prey, before entering an almost vertical climb back up to the warmer waters at the surface.

But the team found that despite the water temperature changing by about 20C over the course of the dive, the shark’s body temperature remained stable for most of it. They believe that the only way the shark can maintain a high body temperature over the course of these dives is to stop the cold water flowing over its gills, which would involve a high level of heat exchange and rapidly cool the animal down.

“Essentially, the sharks seem to be holding their breath. We believe that the breath-holding mechanism is the closing of the gill slits,” said researchers.

The study was published in the journal Science.