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Nascent Drama

Even in oxygen-rich water, a net will impair a shark's movement so it can't breathe.
 
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It might seem tiring to us humans to think about any sort of perpetual movement as a way to survive; we all like crashing on the couch every now and then.
 
But as it turns out, it's more work for these sharks to remain still than it is to swim.
 
In a study of lemon sharks, which switch between breathing methods, juveniles breathed 6 percent more efficiently when moving than when resting,
 
even when resting so that the current allowed the water to flow directly into their mouths [source: Morrissey and Gruber].
 
This fact may help to explain what happens when sharks face the danger of hypoxia, or a deficiency of available oxygen.
 
Sharks that breathe by buccal pumping increase the force of the pumping to try to bring in more oxygen while reducing their other activity to conserve energy.
 
Obligate ram breathers, however, increase their energy, swimming faster and opening their mouth wider [source: Carlson and Parsons].
 
It may seem counterintuitive to speed up when faced with less oxygen, but that just may be more energy efficient for these sharks.
 
That's not to say these sharks don't catch a break every now and then.
 
For obvious reasons, it can be hard to keep track of a shark that's constantly swimming, so it's difficult for scientists to know how or when they rest.
 
An experiment with a small shark, the spiny dogfish, indicated that swimming is coordinated by the spinal cord,
 
not by the brain, so sharks may be able to shut down their brain and rest while still swimming [source: Martin].
 
Sharks in need of some rest may also take advantage of the factors that affect the amount of oxygen in the water, such as salinity, temperature and even time of day.
 
In the 1970s, scientists investigated what came to be known as the Caves of the Sleeping Sharks in Isla Mujeres, Mexico.
 
Inside the caves were motionless reef sharks, which are normally obligate ram ventilators.
 
The scientists determined that the water in the caves had an extremely high amount of oxygen and reduced salinity.
 
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