This shark can hold its breath to stay warm in the deep, study finds | CBC Radio

In a behaviour never before seen in fish, scientists have discovered that scalloped hammerhead sharks will hold their breath to stay warm as they hunt for squid in frigid waters 1,000 metres below the ocean’s surface.

“I would have never, ever have expected a gill-breathing fish to resort to holding its breath to dive down deep in the ocean,” said Mark Royer.

Royer was part of a team of researchers from the Hawai’i Institute of Marine Biology who made this discovery working with sharks near the Hawaiian island of Oahu.

The study was published this week in the journal Science. 

Scalloped hammerhead sharks live in warm, tropical waters around the world and are ectothermic, which means their body temperature changes with their immediate environment.

But if the shark’s body temperature drops, so does its metabolism and activity level, slowing down the shark and making it a less effective hunter.

Royer said some “high performance” fish, like tuna and mackerel sharks, have specialised anatomy that allows them to warm parts of their body when they hunt in cold water. But there’s no evidence that scalloped hammerheads have this. 

Deep dives

Royer and his team tagged sharks with sensors that allowed them to monitor swimming activity, body temperature and the properties of the environment they swam through, including water temperature and depth.

Despite diving from 26 C to 5 C, which is “about the same as an ice-bath,” according to Royer, the sharks were able to maintain a relatively warm body temperature. 

Royer said cold water should have flown through their gills, causing their body temperature to drop. 

“So for any fish … the detriment for swimming in cold water is that when you have gills, it’s like having a giant radiator strapped to your head,” he said. 

“If your water temperatures are colder than the fish’s body, all of that heat will be dumped into the environment.”

Royer found that the simple solution for the sharks was to just close their gills to prevent the cold water from flowing through. This prevented them from losing body heat.

“When they start to … dive down to the bottom, they close the flaps, shoot straight down very quickly, eat what they can, and when it’s time to go up, they pitch themselves at a steep angle and burst up towards the surface,” Royer said.

The sharks hold their breath for approximately 17 minutes, but Royer says his team isn’t sure how they manage to go without oxygen for so long.

So far, scalloped hammerhead sharks are the only known species to exhibit this behaviour, but Royer thinks other species might use similar strategies.

For example, the oceanic whitetip shark lives in tropical environments, but also dive into deep, cold waters.

“That’s something that needs to be investigated in a further study,” he said. “It’s possible that they have really well-suited physiology and metabolism to be able to do such intense exercise with a lack of oxygen.”

Royer said further research is needed to understand if other animals use similar strategies.

Written and produced by Shazara Khan