Hammerhead sharks hold their breath to stay warm in cold ocean depths

These flat-headed predators typically thrive in warm-temperature coastal and tropical waters.
Mrigakshi Dixit
A hammerhead shark
A hammerhead shark


Scalloped hammerhead sharks (Sphyrna lewini) have developed an unusual method of keeping warm while diving deep in cold waters: they hold their breath. 

This surprising discovery was made by researchers at the University of Hawai'i at Manoa. "This was a complete surprise. It was unexpected for sharks to hold their breath to hunt like a diving marine mammal. It is an extraordinary behavior from an incredible animal,” said Mark Royer, lead author and researcher with the Shark Research Group at the Hawai'i Institute of Marine Biology (HIMB), in an official release

Study obtains direct evidence of this behavior

These flat-headed predators typically thrive in warm-temperature coastal and tropical waters. The majority of fish, including hammerhead sharks, are ectothermic, which means their surroundings control their internal body temperature. 

However, these species have been observed repeatedly diving to cold ocean depths (800 meters) in search of prey, like fish and squid. The seawater temperature at these depths is usually low, and they must keep their bodies warm to survive.

The team attached sensor devices near the dorsal fins of six hammerheads near Hawaii to better understand their breeding strategy in deep cold water. As per the study, the individual devices helped to gather data on muscle temperature, ocean depth, body orientation, and activity levels. 

The device measurement demonstrated that the shark's muscle remained warm throughout its dive into deep cold water. When hammerhead sharks approached their natural habitat (surface water), the muscles started to cool again. 

Their gills, which allow them to breathe, are said to be significant heat loss points. The team also recorded videos of sharks diving into cold water to obtain direct evidence. The video of a shark swimming to a depth of 1,044 meters found "gill slits" were tightly closed during this time. Similar images of sharks swimming near the surface of the water showed their gill slits wide open. 

Even the computer modeling found that sharks could prevent heat loss from their gills, which kept their bodies warm during the deep dive.

All these results validate that sharks hold their breath for a short duration which helps them to withstand frigid temperatures at the ocean depths.

"Holding their breath keeps scalloped hammerhead sharks warm and shuts off their oxygen supply. So, although these sharks hold their breath for an average of 17 minutes, they only spend an average of four minutes at the bottom of their dives at extreme depths before quickly returning to warmer, well-oxygenated surface waters where breathing resumes,” explained Royer. 

According to the researchers, the shark may have evolved this trait to hunt deep-dwelling prey. Up next, the team wants to focus on understanding the hammerheads’ metabolism to unravel more deeds on how they cope with frequent cold plunges.

The results have been published in the journal Science.

Study abstract:

Fish moving between different thermal environments experience heat exchange via conduction through the body wall and convection from blood flow across the gills. We report a strategy of preventing convective heat loss at the gills during excursions into deep, cold water by the tropical scalloped hammerhead shark (Sphryna lewini). Adult scalloped hammerhead sharks dive rapidly and repeatedly from warm (~26°C) surface waters to depths exceeding 800 meters with temperatures as low as 5°C. Biologgers attached to adult sharks show that warm muscle temperatures were maintained throughout the deepest portion of each dive. Substantive cooling only occurred during the latter stages of the ascent phase and, once initiated, was rapid. Heat transfer coefficient modeling indicated that convective heat transfer was suspended, probably by suppressing gill function during deep dives. This previously unobserved strategy has broad similarities to marine mammal “breath hold” diving

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