Sharks apparently sleep, even with their eyes wide open | Smart News

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Checkerboard sharks apparently sleep with their eyes open or closed depending on the amount of surrounding light.
Victorgrigas via Wikimedia Commons

Sharks can sleep and often choose to keep their eyes open while they do, according to a new study published in Biology Letters.

Because some sharks have to swim constantly to keep oxygen-rich water flowing over their gills, it has long been said that they don’t sleep at all. Scientists in Australia have now documented a species of dormant bottom-dwelling shark for the first time, upending the long-running debate.

“Until now, shark sleep has been completely unstudied and unknown,” study author Michael Kelly, an ecophysiologist at La Trobe University, tells Robyn White for Newsweek. “Sharks are a particularly important group because they are the oldest living jawed vertebrates – a trait they share with us.”

In their study, the team of scientists observed checkerboard sharks, a nocturnal shark native to New Zealand, apparently asleep during rest periods. Unlike great white sharks and tiger sharks, which must keep swimming to ventilate their gills, checkerboard sharks are a species of mouth-pump sharks, which manually push water over their gills to take in oxygen at the ‘stop.

To see if the animals were actually asleep, the team analyzed the metabolism and posture of seven checkerboard sharks over 24 hours. When the sharks rested for five minutes or more, their oxygen consumption dropped, suggesting the animals fell asleep.

“We know that a drop in metabolism is a telltale sign of sleep in many, many other animals,” Kelly told the GuardianIt’s Donna Lu.

Because checkerboard sharks are ambush predators, the three-foot-long animals typically position themselves with their fins up and their heads held high. But when the sharks fell asleep, the researchers noticed that the fish changed posture by flattening out and snuggling closer to the ground.

The sharks sometimes slept with their eyes closed, but mostly during the day. At night, they chose to keep their eyes open more often, leading researchers to suspect that the sharks’ eye closure may have more to do with light than the sleep state itself, according to Scientific alertThis is Tessa Koumoundouros. About 38% of sharks kept their eyes open at night, even when they appeared to be asleep.

“We have provided the first physiological evidence for sleep in sharks,” the team wrote in their paper.

The work builds on an earlier study by the same research team that investigated the behavioral cues of sleeping sharks. In this study, they found that it was harder to get the sharks to move if they had remained still for a long time, suggesting that the animals were sleeping, reports Veronique Greenwood for the New York Times.

“Not only do sleeping sharks have reduced responsiveness to stimulation, they also have a lower metabolic rate,” the authors note in their study.

The team notes that further research will be needed to see if other species sleep like the checkerboard shark. Next, they plan to analyze the brain activity of the sharks while they sleep to learn more about their awake and resting states.

“[Sharks] have been swimming in our seas for over 400 million years and have evolved very little during that time, giving us a glimpse into the past,” Kelly said. Newsweek. “Understanding how and why these animals sleep will provide important insight into sleep function and how it changes over time.”

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