A new report has found hammerhead sharks "hold their breath" during dives to regulate body temperature – an adaptation that may help them better cope with declining oxygen saturation levels caused by global warming.
A paper published in the journal Science reports that when hammerhead sharks are diving to depths of 800m and water temperatures of only 5oC they close their gill slits and mouth to delay an onset of a reduction in body temperatures.
Adjunct Senior Research Fellow Dr Mark Meekan, from The University of Western Australia's Oceans Institute, said during the day hammerhead sharks descend to depths of 500 to 1000m to feed.
"Hammerhead sharks, which feed on squid, must descend to these cold dark waters," Dr Meekan said.
"This presents a major physiological challenge for these sharks, whose primary habitats are relatively warm surface waters."
Fish gills function to extract oxygen from surrounding water, but they are also very efficient at transferring heat.
"Hammerhead sharks are not warm-blooded like mammals, which means that their body temperatures, metabolism and swimming speeds are set by the temperatures of the water they inhabit," he said.
"This is fine in shallow warm water, but to avoid rapid cooling at depth and remain as fast-moving hunters, the sharks close the gill slits and the mouth to stop water flow and avoid temperature loss — effectively, they are holding their breath, somewhat like a freediver or a whale.
"On ascent, the shark can open the gill slits and start the flow of water across the gill to oxygenate the blood, but this also starts to cool the body. Once in warm surface water again, the gills can transfer heat to rapidly warm blood and vital organs."
Across the world's oceans, global warming is driving a decline of dissolved oxygen at depth. Although these areas of extremely low oxygen saturation present a major threat to most smaller species, the ability of hammerhead sharks to cease respiration while foraging at depth could mean that the species is more likely to cope in these changing environments.
"Sharks have been in the world's oceans for an exceptionally long time – around 400 million years," Dr Meekan said. "Studies like this give us insights into how they have managed such extraordinary persistence across the changing ocean environments they have faced, and how they might be able to cope with the oceans of the future."
Dr Meekan co-authored this commentary piece while Senior Research Scientist at the Australian Institute of Marine Science.
