While deep diving into chilly water to hunt prey like deep sea squids, scalloped hammerhead sharks hold their breath to keep their bodies warm. This discovery, which was made by researchers at the University of Hawaii at Manoa and published today in Science, offers crucial new insights into the physiology and ecology of a species that bridges the gap between deep and shallow water habitats.
“This was a complete surprise!” said Mark Royer, lead author and researcher with the Shark Research Group at the Hawai’i Institute of Marine Biology (HIMB) in the UH Manoa School of Ocean and Earth Science and Technology. “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.”
Shark gills are natural radiators that, if scalloped hammerhead sharks did not seal their gill slits during deep dives into cold water, would quickly cool the blood, muscles, and organs. Although they live in warm waters, these sharks must maintain body heat in order to hunt successfully in depths where seawater temperatures are similar to those in Kodiak, Alaska (about 5oC/40oF).
“Although it is obvious that air-breathing marine mammals hold their breath while diving, we did not expect to see sharks exhibiting similar behavior,” said Royer. “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.”
This new and detailed understanding of scalloped hammerhead physiology and ecology 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 large-scale fishing in the mesopelagic ‘twilight zone,’ both of which might make it harder or more dangerous for these sharks to hunt their natural prey.
Mark Royer
By giving deep-diving scalloped hammerhead sharks instruments that simultaneously assessed their muscle temperature, depth, body orientation, and activity levels, the research team was able to unearth this surprising phenomena. When the sharks surfaced toward the conclusion of each dive, they observed that their muscles had remained warm throughout the whole plunge into the chilly depths.
In order to keep their bodies warm during these deep dives into frigid water, hammerhead sharks may be reducing heat loss from their gills, according to computer models.
The gill slits of a scalloped hammerhead shark moving along the seabed at a depth of 1,044 meters (more than 3,400 feet) were securely closed in the video, in contrast to similar photographs of these sharks traveling in surface waters where their gill slits were wide open.
When scalloped hammerhead sharks surface at the end of each dive, there is a sudden drop in muscle temperature, which suggests that they 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,” said Royer. “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.”
“This discovery fundamentally advances our understanding of how scalloped hammerhead sharks are able to dive to great depths and withstand frigid temperatures in order to capture prey,” said Royer. “It also demonstrates the delicate physiological balance that scalloped hammerhead sharks must strike in order to forage 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 nursery habitat loss.
“This new and detailed understanding of scalloped hammerhead physiology and ecology 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 large-scale fishing in the mesopelagic ‘twilight zone,’ both of which might make it harder or more dangerous for these sharks to hunt their natural prey,” said Royer.
“This extraordinary physiological feat that allows scalloped hammerhead sharks to expand their ecological niche into the deep sea could very well make them vulnerable to additional human impacts.”
