Scalloped hammerhead sharks swim through the waters off the Kona coast. CORY FULTS photo
Scalloped hammerhead sharks hold their breath to keep their bodies warm during deep dives into cold water where they hunt prey such as deep sea squids, according to a study published Thursday in the journal Science.
The discovery by University of Hawaii at Manoa researchers provides insights into a species that serves as an important link between the deep and shallow water habitats.
Mark Royer, lead author and researcher with the Shark Research Group at the Hawaii Institute of Marine Biology in the UH-Manoa School of Ocean and Earth Science and Technology, called it “a complete surprise.”
“Although it is obvious that air-breathing marine mammals hold their breath while diving, we did not expect to see sharks exhibiting similar behavior,” Royer said in a news release Thursday. “This previously unobserved behavior reveals that scalloped hammerhead sharks have feeding strategies that are broadly similar to those of some marine mammals, like pilot whales. Both have evolved to exploit deep dwelling prey and do so by holding their breath to access these physically challenging environments for short periods.”
Shark gills are natural radiators that would rapidly cool the blood, muscles and organs if scalloped hammerhead sharks did not close their gill slits during deep dives into cold water. These sharks are warm-water animals but feed at depths where seawater temperatures are similar to those found in Kodiak Alaska (around 40 degrees Fahrenheit), yet they need to keep their bodies warm in order to hunt effectively.
Researchers equipped deep-diving scalloped hammerhead sharks with devices that simultaneously measured their muscle temperature, depth, body orientation and activity levels. They saw that their muscles stayed warm throughout their dive into deep cold water but suddenly cooled as the sharks approached the surface toward the end of each dive. Computer modeling suggested that hammerhead sharks must be preventing heat loss from their gills to keep their bodies warm during these deep dives into cold water.
Additionally, video of a scalloped hammerhead shark swimming along the seabed at a depth of 1,044 meters (more than 3,400 feet) showed its gill slits tightly closed, whereas 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 suggests that they opened their gill slits to resume breathing while still in relatively cool water.
The sharks hold their breath for an average of 17 minutes but only spend an average of four minutes at the bottom of their drives at extreme depths before they return to the warmer, well-oxygenated surface, Royer explained.
He said the discovery can help in conserving the 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 large-scale fishing.”
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 nursery habitat loss, according to UH.
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