These Sharks Don't Have to Keep Moving to Stay Alive: 'Blew Our Minds'

Despite the popular belief that all sharks need to keep swimming or they'll die, many species of shark can get along just fine staying still.

Gray reef sharks are the most recent species now recognized as not needing to stay constantly on the move, according to new research in the journal Fish Biology.

These endangered sharks were previously thought to need to always move to maintain the flow of water over their gills, but the research reveals that the little sharks were spotted chilling out motionless on a reef off the Seychelles.

"On routine survey dives around D'Arros we found gray reef sharks resting under coral reef ledges. This is significant because this is not something we believed they could do," said Robert Bullock, the director of research at the Save Our Seas Foundation D'Arros Research Center and a co-author of the paper, in a statement.

The discovery was "a pure chance encounter that blew our minds," James Lea, study co-author and CEO of the foundation, said in the statement.

Sharks are often falsely assumed to need to constantly move, as this keeps a fresh supply of oxygenated water flowing over their gills, allowing them to absorb the oxygen into their bloodstream. This is called obligate ram ventilation, as the water needs to be constantly forced over the gills through their movement through the sea.

Other species of fish don't need to do this. Instead, they can open and close their mouths to force the water over their gills, which allows them to stop and take a break.

Bullock told Newsweek, "Ram ventilation is the process by which many sharks breathe. It simply means that as the animal swims forward, it holds its mouth open and this allows water to pass over their gills. Some sharks, however, are capable of breathing whilst lying still through a process called buccal pumping."

He went on: "This is essentially the opening and closing of the mouth to force water across the gills. Whether a shark can buccal pump or not depends on its lifestyle. Less active sharks, such as nurse sharks, are able to buccal pump, for example. More active sharks use ram ventilation to breathe."

More and more sharks once thought to be obligate ram ventilators are now being identified as buccal pumpers. Until now, gray reef sharks were assumed to ram ventilate, based on their body shape and size, and the habits of their close cousin species.

However, the research shows that the sharks can rest and ventilate their gills for at least 40 minutes. All the resting sharks were facing in different directions, which implies that they weren't all facing into a current of water and therefore must have been pumping the water over their gills manually.

"The gray reef shark is a species we believed for a very long time to have belonged to a group of obligate ram ventilating sharks, meaning that they must keep moving in order to breathe. This discovery changes our understanding," Bullock said. "Our finding not only shows these sharks laying still for prolonged periods but we also see the movements of the mouth indicative of buccal pumping, something we had no idea they could do."

This discovery may also shed light on how and when sharks sleep. There is no clear evidence that ram ventilator sharks actually sleep at all, with some theories suggesting that they sleep with half their brain, like dolphins and sperm whales, or sleep facing into ocean currents. Because the gray reef sharks didn't respond to the divers watching them rest, they may have been sleeping.

"I love things that challenge our current thinking, and I've always thought of gray reef sharks as a clear example of a shark species that needs to swim to breathe. Clearly not from this discovery," Lea said.

He went on: "Sharks and sleep are two are my favourite things. And there's always room for more of both. It raises all kinds of other questions. How are they coping? For how long do they sleep? How often? We have so much to learn still, and to me that's really exciting."

The researchers hope that more of the sharks' secrets will be revealed and, as a result, bring the public closer to a little-considered endangered species. Gray reef sharks are a smaller species of shark, usually less than 6 or 7 feet long, found in the coastal Indo-Pacific region. They are listed as endangered because of their population declines, mostly due to the effects of overfishing.

'It's cool because this is a well-studied species and we are working hard to uncover more intricate things about its biology, physiology, behaviour and ecology—and yet an observation like this can come along to remind us just how much we still don't know," Bullock said.

He continued: "Conservation is founded on a thorough scientific understanding, so interesting discoveries such as this always have the potential to inform and interact with conservation in the future. In this instance in particular, this update to our understanding of how gray reef sharks breathe could help us to better interpret this species' physiological responses to environmental change."

This may help us protect these sharks and others like them around the world against the encroaching effects of climate change.

Lea said, "For the conservation of sharks, it's really key to apply our understanding of how they use their environment, and also how this may change in response to shifts in environmental conditions.

"How important is this rest, or possible sleep, for the sharks? And what's the impact on them if they can't get that rest if conditions change—if oxygen levels, for example, rise or fall—due to climate change?" he asked.

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Update 12/19/23, 12:45 a.m. ET: This story was updated with comments from Robert Bullock.