Ah, sleep.
It accounts for one-quarter to one-third of the human lifespan and is an essential function that allows our bodies and minds to recharge, leaving us refreshed and alert when we wake up. Sleep is found across the animal kingdom, although it isn’t well-studied for many species. Take sharks, for example. While it is well known that some species can rest on the bottom floor, scientific evidence of sharks sleeping has only been documented in Australia recently – and for the first time.
Scuba diver comments of draughtsboard sharks (Cephaloscyllium isabellum) sleeping reached the ears of the study’s lead author, Dr. Michael Kelly of La Trobe University, who set out to see if this was true or not. “Recent studies have found that Port Jackson (Heterodontus portusjacksoni) and draughtsboard sharks are nocturnal with a reduced responsiveness to stimulation while asleep,” the authors explain. “However, as sleep is both a behavioural and physiological state involving multiple components, including changes in eye state, muscle tone, brain activity and metabolism, it is necessary to investigate as many sleep components as possible to fully characterize the sleep state, or states, in sharks.”
Draughtboard shark, also known as the Australian swellshark (Cephaloscyllium laticeps), resting… … [+]
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Draughtboard sharks are a stocky, benthic species that is commonly observed on the continental shelf and continental slope, but is sometimes seen by oceangoers in relatively shallow waters. Known to increase their stomach with air or water when disturbed, these sharks are often seen lying on the ocean floor with their mouths opening and closing to “inhale” water. This is because they are buccal pumping sharks, which manually push water over their gills so they can breathe, even while still. The researchers collected seven draughtsboard sharks from Hauraki Gulf in the north-eastern part of New Zealand, and housed them in an outdoor aquarium under natural light conditions. Monitoring them for a 24-hour period, they assessed changes in metabolic rate (measured through oxygen consumption) and behaviours associated with sleep in other animals such as their body posture or whether eyes were open or closed.
Once the was data collected, the scientists set out to determine whether sleep plays a role in energy conservation in sharks… and found that the sharks’ metabolism dropped when the sharks were inactive for longer than five minutes. “We know that a drop in metabolism is a telltale sign of sleep in many, many other animals,” Kelly told The Guardian. And while the “eye closure is a poorer indication of sleep” for these sharks, they did change their body posture to lying flat while sleeping.
However, some sharks and bony fishes can’t accomplish buccal pumping and have to swim continuously to maintain the water/blood gas exchange (called ram ventilation). So what about those species, like great white sharks (Carcharodon carcharias)? “The idea was … those animals aren’t sleeping because they move continuously,” Kelly said, adding the theory had been debunked in other animals. “Staying still is not a criteria for sleep. We see that in marine mammals – whales and dolphins are able to keep swimming and sleep … We know that birds are able to keep flying and sleep.”
Kelly plans to take this research further by monitoring the draughtsboard shark’s brain activity, and the whole team is excited to do further research on sleep in ram ventilators.
