Grey Reef Sharks Found Resting, Rewriting What We Know Them

Predators in perpetual motion, sleepless in our seas.

If that’s your image of sharks, you are not alone. And for good reason: we once believes that sharks must keep swimming to breathe. Since then, scientists have revealed multiple ways through which these animals extract life-sustaining oxygen from seawater. Some sharks, known as obligate ram ventilators, ‘ram’ oxygen-rich seawater over their gills and are required to maintain constant motion to do so; species like hammerheads, makos, threshers, salmon (Lamna ditropis) and great whites (Carcharodon carcharias). On the other hand, there are buccal pumpers, that actively pump seawater over their gills even when stationary; these includes species like nurse and bullhead sharks.

As of right now, there is no solid evidence of sleep behavior in ram ventilators. Researchers speculate that ram-ventilating sharks might sleep using only half of their brain (similar to sperm whales and bottlenose dolphins), or they may adopt a strategy of passive breathing by positioning themselves to face into ocean currents. But recent findings of routine survey dives around D’Arros might shed some light on the resting behavior of one species of ram ventilator - grey reef sharks (Carcharhinus amblyrhynchos). Recognized for its slender, streamlined body and distinctive gray coloration, this species belongs to the Carcharhinidae family. Inhabiting coral reef ecosystems in the Indo-Pacific region, they play a crucial role in maintaining the ecological balance of the habitat by controlling the abundance of prey. Unfortunately, these sharks face conservation concerns due to habitat degradation, overfishing, and climate change, warranting ongoing efforts to protect and preserve their vulnerable populations.

“On routine survey dives around D’Arros, we found grey reef sharks resting under coral reef ledges. This is not something we believed they could do,” explains Dr. Robert Bullock, the Director of Research at the Save Our Seas D'Arros Research Centre in Seychelles. “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.” Notably, the sharks rested facing in all directions, dismissing the idea that they would only rest when facing into ocean currents. The sharks also remained still, except for lower jaw movements that suggest these ram-ventilating sharks can switch to buccal pumping behaviour. Craig Foster, founder of the SeaChange Project, expressed the unique experience of encountering sleeping sharks, saying, “There is something very special about ‘tiptoeing’ around underwater at a depth of 82 feet (25 meters) and looking into the open eyes of sleeping sharks, moving carefully so as not to wake the peaceful beauties.”

Dr. James Lea, the CEO of the Save Our Seas Foundation, is enthusiastic about the implications of the discovery. “This raises all kinds of other questions,” he says. “How are they coping? How long for? How often? We have so much to learn still, and to me, that's exciting.” If the grey reef shark, once believed to require constant motion to breathe, can switch its breathing mode and rest, there is undoubtedly more to uncover about other ram-ventilation sharks. The authors unanimously agree that this discovery not only sheds light on the mysteries that still surround sharks, but also prompts contemplation about their relationship with the natural world. “It’s key to understanding how they use their environment and also how this may change in response to shifts in environmental conditions,” questions 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,” agrees Bullock. “Science is about being wrong quite a lot. And that’s OK.”