Grey reef sharks can stop and rest, which was thought to be impossible

A study on grey reef sharks in the Seychelles is challenging long-held beliefs about shark breathing and sleeping patterns. 

The research contradicts the long-held belief that some sharks, known as obligate ram ventilators, must constantly swim to breathe. 

Science of shark breathing

Traditionally, experts believed that sharks employed two primary methods of respiration: ram ventilation and buccal pumping. 

Ram ventilators, like the grey reef shark, were thought to require constant movement to force water over their gills. 

By contrast, buccal pumpers can actively pump water while stationary. This distinction has significant implications for how these creatures rest and potentially sleep. 

Surprising discovery 

“On routine survey dives around D’Arros we found grey reef sharks resting under coral reef ledges,” said Dr. Robert Bullock, the director of research at the Save Our Seas D’Arros Research Centre (SOSF-DRC).

The team observed the sharks resting alone and in groups, showing signs of what could be considered sleep. 

“This is not something we believed they could do. The grey reef shark has been considered a ram-ventilating species, unable to rest, so to find these ones resting turns our fundamental understanding of them on its head,” said Dr. Bullock.

Rethinking shark sleep

The sharks demonstrated lower jaw movements, suggesting a shift from ram ventilation to buccal pumping. This is a remarkable adaptation for a species that was previously believed to be incapable of such behavior. 

“There is something very special about ‘tiptoeing’ around underwater at a depth of 25 meters and looking into the open eyes of sleeping sharks, moving carefully so as not to wake the peaceful beauties,” said Craig Foster, founder of the SeaChange Project.

“I love things that challenge our current thinking, and I’ve always thought of the grey reef shark as a clear example of a species that needs to swim to breathe. Clearly not from this discovery!”

New questions emerge

The research presents essential questions about the resting and potential sleeping patterns of sharks, particularly those thought to be obligate ram ventilators. 

“This raises all kinds of other questions. How are they coping? How long for? How often? We have so much to learn still, and to me, that’s exciting,” said Dr. James Lea, CEO of the Save Our Seas Foundation.

Study implications 

The researchers plan to explore how these behaviors impact the sharks’ use of their environment. They noted that if the grey reef shark can switch its mode of breathing, and rest, there’s more to uncover about similar sharks. 

“It’s key to understanding how they use their environment and also how this may change in response to shifts in environmental conditions,” explained Dr Lea. 

“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, such as oxygen levels rising or falling due to a changing climate?”

“I hope that these findings serve as a reminder of how much we still do not know and how exciting that is,” said Dr. Bullock. “Science is about being wrong quite a lot. And that’s OK.”

The study is published in the Journal of Fish Biology.

Video Credit: Filmed by Craig Foster | © Save Our Seas Foundation

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