The discovery puts the shark on the short list of vertebrates that can tolerate situations where there is zero to very little available oxygen. In addition to the shark, these include the crucian carp, freshwater turtles and leopard frogs.
For the shark, breathing can become a challenge when the sun goes down.
"The epaulette shark lives on shallow parts of the Great Barrier Reef where hypoxia (low oxygen levels) is common at night, particularly during low tide when their habitat may become cut off from the ocean," explained Goran Nilsson, who worked on the study.
Nilsson, a professor in the Department of Molecular Biosciences at the University of Oslo, and his colleagues made the determination through use of a non-invasive technique for studying one indicator of shark-produced electricity: eye electrical activity.
The researchers first anesthetized the sharks, captured by hand at Australia's Great Barrier Reef, and acclimated them to a seawater tank at the nearby Heron Island Research Station. The scientists lowered the oxygen levels in the tank and lightly touched an electrode to the shark's eyes while shining a light into them. The measured response of various energy waves emitted by the eyes revealed the shark's electricity output in terms of vision.
The scientists found that the epaulette shark completely shuts down the response of nerve cells in the retina, or the light-sensitive membrane that lines the inner eyeball. These cells normally transfer information to the visual nerve so, when outside oxygen levels are low, the shark essentially goes blind.
Since the eyes represent just one aspect of shark electricity production, it's believed that the spotted fish probably shuts down other parts of its central nervous system. Electrical activity accounts for 50 percent or more of nerve energy consumption, so reducing it allows the shark to tolerate low oxygen conditions.
Humans, in contrast, may pass out if they have trouble breathing, but this is often a severe shut down that could lead to even more problems. The epaulette shark fully recovers from its downtime state.
The researchers, whose paper has been accepted for publication in the journal Comparative Biochemistry and Physiology, Part A, conducted the same eye test on freshwater turtles and leopard frogs. The turtles actually beat out both the sharks and the frogs in their ability to tolerate zero to low oxygen.
The turtles didn't lose their eyesight and hardly changed at all, despite some reduced brain activity. Prior studies show crucian carp also have an incredible tolerance for conditions that would suffocate nearly all other animals.
Nilsson explained that these species evolved the David Blaine-like seemingly magical ability in order "to allow overwintering at the bottom of ice-covered ponds, small lakes and streams."
He added, "This habitat can become free of oxygen for several months during the winter."
Les Buck, associate professor in the Department of Cell & Systems Biology at the University of Toronto, told Discovery News that the new study "is interesting" particularly because it demonstrates that "visual information can be sacrificed in the severely hypoxic situation."
Since the epaulette shark has evolved its low oxygen tolerance at a temperature close to that of the human body, Nilsson said studies on the shark "could give us additional information on strategies that animals have evolved" to cope with natural happenings akin to what occurs when a person suffers a stroke, a heart attack or certain complications related to diabetes, when breathing or low blood flow present potentially life-threatening physical challenges.
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