You may have heard that all sharks, if not all fish, must constantly move to maintain a constant flow of water and oxygen through their gills. While most fish can stop for a break and open and close their mouths to keep the water flowing, some species really need to keep going. These are called forced ventilation devices. Unlike their more sedentary relatives, they cannot use their mouths to pump water through their gills while at rest.

Biologists evaluated the gray reef shark (Carcharhinus amblyrhynchos) One of them. But during routine surveys by Save Our Seas divers, these sharks were captured napping on the seafloor in a shelter on rocky ledges surrounding the Seychelles islands.

If gray reef sharks were obligate ventilators, such sleep would be fatal, especially since the water was barely moving during some observations.

Although the sharks were motionless, divers observed their lower jaws rising and falling, similar to species that use the buccal pumping system (opening and closing motion) to “breathe”; during rest. Filmmaker Craig Foster, known for the 2020 documentary My Octopus Teacherwas one of these divers and the author of the article reporting the observation.

“It’s a very special thing to tiptoe about 25 meters underwater and look into the open eyes of sleeping sharks, moving carefully so as not to wake up the peaceful beauty,” he says.

“I love things that challenge our current way of thinking, and I have always thought of the gray reef shark as a prime example of a species that must swim to breathe. Obviously not from this discovery. “The newspaper reports that sleeping gray reef sharks have been spotted in two different locations. In September 2022, three sharks (at least one female) were observed napping together for the first time.

The fact that the three sharks were facing different directions was a big clue that they may have been buccal pump breathing; If they trusted the current, they would be looking in the same direction.

During a return visit to the area in April 2023, divers saw between one and four sharks resting on this underwater bed. The team also cites an observation by marine photographer Luke Saddler, who recorded a shark napping under a large coral head using an iPhone 13 elsewhere in early 2023.

During all of these observations, divers stayed up to 5 meters (16 feet) away from the sharks and spent 15 to 40 minutes nearby to minimize disturbance. While divers watched the beating of their jaws and the random movements of their always-open eyes (perhaps the shark dreams of underwater sheep), the sharks were none the wiser as they slumbered.

For gray reef sharks, this is not what they thought was possible, says lead author and marine biologist Robert Bullock, who led the study at the Save Our Seas D’Arros Research Center in the Seychelles.

“During routine research dives around D’Arros, we found gray reef sharks resting under coral reef ledges,” says Bullock. “The gray reef shark was thought to be a ram-breathing, non-resting species, so the discovery of these types of resters overturns our basic understanding of them.”

Shark species are well researched, and scientists often focus on uncovering the finer things about their biology, physiology, behavior and ecology. But Bullock says this observation is a reminder of how much we still don’t know.

“Conservation relies on deep scientific understanding, so exciting discoveries like this can always inform and engage conservation in the future,” he says. “In this case in particular, updating our understanding of how gray reef sharks breathe may help us better interpret this species’ physiological responses to environmental changes.”