Hammerhead sharks hold their breath on deep water hunts to stay warm
by University of Hawaii at Manoa
Published June 28, 2023

Scalloped hammerhead sharks off the coast of the Big Island of Hawai'i. Image by Cory Fults

Newswise — Scalloped hammerhead sharks retain their respiration to maintain their body heat while plunging into chilly waters to pursue prey, such as deep-sea squids. This finding, released today in Science by scientists from the University of Hawai‘i at Mānoa, presents crucial novel perspectives on the physiology and ecology of a species that acts as a vital connection between the deep and superficial aquatic environments.

Mark Royer, the primary investigator and a member of the Shark Research Group at the Hawai‘i Institute of Marine Biology (HIMB) within the UH Mānoa School of Ocean and Earth Science and Technology, expressed, "This came as a total surprise! Sharks retaining their respiration to prey like a diving marine mammal was unforeseen. It is an exceptional conduct exhibited by an astounding creature."

The gills of sharks serve as innate coolers that would swiftly decrease the temperature of the blood, muscles, and organs if scalloped hammerhead sharks did not shut their gill slits while plunging into frigid waters. Although these sharks thrive in warm water, they forage in depths where seawater temperatures resemble those in Kodiak, Alaska (approximately 5ºC/40ºF). Nonetheless, to hunt successfully, they must sustain their body warmth.

Royer stated, "Although it's clear that air-breathing marine mammals retain their respiration while diving, we didn't anticipate observing sharks exhibiting analogous behavior. This behavior, which was previously unobserved, discloses that scalloped hammerhead sharks possess feeding tactics that are similar to those of certain marine mammals, such as pilot whales. Both species have adapted to prey on deep-sea organisms and accomplish this by retaining their respiration to access these demanding environments for brief periods."

The scientific group detected this surprising occurrence by outfitting deep-diving scalloped hammerhead sharks with gadgets that concurrently assessed their muscle temperature, depth, body position, and levels of activity. They observed that the sharks' muscles remained warm during their plunge into chilly water but abruptly cooled as they neared the surface at the end of each dive. Computational simulations indicated that hammerhead sharks had to obstruct heat loss from their gills to maintain their body warmth while undertaking these deep immersions into frigid water.

Furthermore, a video recording of a scalloped hammerhead shark swimming along the ocean floor at a depth of 1,044 meters (over 3,400 feet) displayed its gill slits sealed tightly shut, while comparable footage captured in surface waters exhibited these sharks swimming with their gill slits wide open. An abrupt decrease in muscle temperature as scalloped hammerhead sharks approach the surface at the end of each dive proposes that they reopen their gill slits to resume breathing while still submerged in moderately chilly water.

Royer noted, "Retaining their respiration maintains the body warmth of scalloped hammerhead sharks, but also halts their oxygen supply. As a result, even though these sharks hold their breath for around 17 minutes, they only spend about four minutes at the bottom of their deep dives before rapidly returning to the warmer, oxygen-rich surface waters where they can breathe again."

Royer remarked, "This finding essentially enhances our comprehension of how scalloped hammerhead sharks can plunge into substantial depths and endure icy temperatures to seize prey. Moreover, it exemplifies the intricate physiological equilibrium that scalloped hammerhead sharks need to achieve to forage productively."

Although scalloped hammerhead sharks are not classified as threatened in Hawaiʻi, they are considered regionally endangered in other regions of the world as a result of overfishing, incidental catches, and the decline of nursery habitats.

Royer stated, "This comprehensive and novel comprehension of scalloped hammerhead physiology and ecology advances our capability to effectively preserve and manage this emblematic species by uncovering potential susceptibilities linked with changing oceanic circumstances 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 could make it more difficult or hazardous for these sharks to pursue their natural prey. This extraordinary physiological achievement that enables scalloped hammerhead sharks to expand their ecological niche into the deep sea may also render them more susceptible to further human impacts."