Sharks can literally sleep with their eyes open

A new study from the University of Auckland is providing the first physiological evidence of how and why sharks sleep. The experts confirmed that sharks sleep with a flat and rigid body posture – and often do so with their eyes wide open.

For the investigation, researchers from New Zealand and Australia observed draughtsboard sharks in the Hauraki Gulf off the coast of Auckland. Draughtsboard sharks, also known as the carpet sharks, are known to frequently lie stationary on the seafloor.

Beyond observing the sharks, the team measured their metabolic rate to analyze how much energy they are burning in a restful state. 

“Sleep is a ubiquitous behavior found across the animal kingdom, which is typically characterized by sustained immobility and reduced responsiveness. Despite the vulnerability inherent with sleeping, its persistence across evolutionary time suggests it serves one or more core functions,” wrote the study authors.

“One hypothesis for such a core function is that sleep serves to conserve energy through enforcing restfulness and lowering metabolic rate relative to wakefulness. Energy savings during sleep have been reported in diverse animals, including humans, cats, rats, birds and fruit flies. It is unknown, however, whether reduced energy expenditure also occurs in sleeping fishes.”

The researchers explained that sharks represent the earliest group of jawed vertebrates, so they could potentially provide insight into the evolution of sleep among vertebrates.

“This rationale is particularly salient following the recent discovery of two sleep states in teleosts and in at least two species of lizard that in some respects resemble mammalian and avian non-rapid eye movement and REM sleep,” noted the study authors. 

“The existence of two sleep states in birds and mammals suggests that each state performs a different, but perhaps complementary, function. Any homology between the multiple sleep states observed in ectothermic vertebrates to that of endothermic vertebrates is unclear.”

According to the researchers, their results show that – like with many vertebrates – sleep in sharks is associated with reduced metabolic rate. 

“Thus, the hypothesis that sleep is important for energy conservation is supported by this study in a primitive vertebrate,” wrote the researchers.

“Sleep is largely unstudied in this diverse group of cartilaginous fishes and future research should focus on other physiological indicators of sleep, such as changes in brain activity, for a more complete portrait of sleep in these vertebrates.”

The study is published in the journal Biology Letters.

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By Chrissy Sexton, Earth.com Staff Writer