Sharks: Do They Sleep, and If So, How?
Metabolism and Posture Reveal Sleep in Sharks
For centuries, scientists have debated whether sharks sleep. Some species, like great whites and tiger sharks, must constantly swim to keep oxygenated water flowing over their gills. This led to the belief that sharks don’t sleep at all.
However, recent research has upended this long-held assumption. Scientists in Australia have documented a species of bottom-dwelling shark, the draughtsboard shark, sleeping for the first time.
Unlike great whites and tiger sharks, draughtsboard sharks are buccal pumping sharks. This means they can manually push water over their gills to take in oxygen while remaining stationary.
To determine if draughtsboard sharks were actually sleeping, the research team analyzed their metabolism and posture over 24 hours. They found that when the sharks were resting for five minutes or longer, their oxygen consumption dropped significantly, suggesting they were drifting off to sleep.
In addition to a drop in metabolism, the researchers also observed changes in the sharks’ posture. When they were asleep, the sharks flattened their bodies and nestled closer to the ground.
Eye Closure and Light
Interestingly, the researchers found that draughtsboard sharks sometimes slept with their eyes shut, but mostly during the day. At night, they opted to keep their eyes open more often.
This led the researchers to suspect that the sharks’ eye closure may have more to do with light than the sleep state itself. Around 38% of sharks kept their eyes open at night, even when they appeared to be asleep.
Evidence of Sleep in Sharks
The combination of a drop in metabolism, changes in posture, and reduced responsiveness to stimuli provides strong evidence that draughtsboard sharks do indeed sleep.
This finding is significant because it challenges the long-held belief that sharks don’t sleep. It also provides new insights into the evolution of sleep, as sharks are the oldest living jawed vertebrates.
Future Research
The research team plans to conduct further studies to investigate sleep in other shark species. They also plan to analyze the sharks’ brain activity while they sleep to learn more about their wake and rest states.
Understanding how and why sharks sleep will provide important insights into the function of sleep and how it has evolved over time.
Additional Information
- Draughtsboard sharks are ambush predators that typically hunt at night.
- They use their camouflage to blend in with the seafloor and wait for prey to come close.
- Draughtsboard sharks are found in coastal waters around New Zealand.
- They are relatively small, reaching a maximum length of about three feet.
- Draughtsboard sharks are not considered to be a threat to humans.
