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Scalloped hammerhead sharks hold their breath to keep their bodies warm while diving deep into the cold waters as they hunt prey such as squid in the deep sea. This discovery was published today in Sciences By the University of Hawaii at Mānoa, researchers provide important new insights into the physiology and ecology of species that serve as an important link between deep and shallow water habitats.
“This was a complete surprise!” said Mark Royer, lead author and researcher in the Shark Research Group at the Hawaii Institute of Marine Biology (HIMB) in the UH Manoa College of Ocean and Earth Sciences and Technology. “It was unexpected for sharks to hold their breath to hunt like a diving marine mammal. It’s extraordinary behavior from an incredible animal.”
Shark gills are natural radiators that would rapidly cool blood, muscles, and organs if scalloped hammerhead sharks had not closed their gill slits during a deep dive in cold water. These sharks are warm water animals but they feed at depths where sea water temperatures are similar to those of the Alaskan Kodiak (about 5 °C / 40 °F), however they need to keep their bodies warm in order to hunt effectively.
“Although it is clear that air-breathing marine mammals hold their breath while diving, we did not expect to see sharks exhibit similar behaviour,” Royer said. “This previously unobserved behavior reveals that scalloped hammerhead sharks have feeding strategies very similar to those of some marine mammals, such as pilot whales. Both have evolved to exploit prey that live in the depths and do so by holding their breath to reach to these physically challenging environments for short periods.”
The research team discovered this unexpected phenomenon by equipping deep-diving scalloped hammerhead sharks with devices that simultaneously measure their muscle temperature, depth, body orientation, and activity levels. They saw that their muscles remained warm throughout their dives into the cold, deep water but suddenly cooled as the sharks approached the surface at the end of each dive. Computer modeling has suggested that hammerhead sharks must prevent heat loss from their gills to keep their bodies warm during deep dives in cold water.
In addition, a video of a scalloped hammerhead shark swimming along the sea floor at a depth of 1,044 meters (over 3,400 feet) showed its gill slits tightly closed, while similar images from surface waters show these sharks swimming with their gill slits wide open. . A sudden cooling in muscle temperature as scalloped hammerhead sharks approach the surface at the end of each dive indicates that they have opened their gill slits to resume breathing while still in relatively cool water.
“Holding their breath keeps scalloped hammerhead sharks warm, but also shuts off their oxygen supply,” Royer said. “So, although these sharks hold their breath for an average of 17 minutes, they spend an average of only four minutes at the bottom of their dives at extreme depths before quickly returning to the warmer, more oxygen-rich surface waters where breathing resumes.”
“This discovery fundamentally advances our understanding of how scalloped hammerhead sharks are able to dive to great depths and withstand freezing temperatures in order to capture prey,” Royer said. It also demonstrates the delicate physiological balance that scalloped hammerhead sharks must strike in order to feed successfully.
Scalloped hammerhead sharks are not listed as threatened in Hawaii but are regionally endangered in other parts of the world due to overfishing, bycatch, and loss of nursery habitat.
“This new and detailed understanding of the physiology and ecology of the scalloped hammerhead enhances our ability to effectively manage and conserve this iconic species by revealing potential vulnerabilities associated with changing ocean conditions or future human exploitation of these deep foraging habitats, such as deep-sea mining or fishing,” Royer said. widespread in the ‘twilight zone’ in the mesopelagic seas, both of which may make it difficult or more dangerous for these sharks to hunt their natural prey.” “This unusual physiological action that allows scalloped hammerhead sharks to expand their ecological niche in the deep sea could make them vulnerable to additional human impacts.”
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