Saturday, August 02, 2008

Shark Avoids Suffocation by Turning Off Electricity

Such an abundance of amazing creatures beneath the ocean surface. Take the epaulette shark and its ability to avoid suffocation by powering down its electrical activity as just one example.

Lack of oxygen can do in most creatures, but a new study has found epaulette sharks have evolved a clever solution for avoiding suffocation -- they shut down their body's electrical activity and even go temporarily blind until they can properly "breathe" oxygen again through their gills.

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.

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