By Charles Q. Choi, Inside Science

It's a myth that all sharks will drown if they stop moving. However, scientists discovered that as the oceans grow more acidic this century, sharks may swim for longer times than before. These new findings suggest that rising atmospheric carbon dioxide levels causing this ocean acidification could significantly disrupt the lives of these predators, which are already in sharp decline globally.

Since the beginning of the Industrial Revolution, the burning of fossil fuels has sharply increased atmospheric carbon dioxide levels, with concentrations of the global warming gas rising from roughly 280 parts per million of the atmosphere in 1750 to about 400 parts per million in 2013. Atmospheric carbon dioxide concentrations will continue to rise, potentially reaching 940 parts per million by 2100 and absorb enough heat to significantly boost average global temperatures.

Oceans absorb carbon dioxide, causing seawater to acidify, and the modern rise of atmospheric carbon dioxide levels has caused the oceans to acidify at a rate far faster than any time in the past 650,000 years. Ocean acidification may cause many coral reefs to dissolve, and prior studies found higher carbon dioxide levels can also disrupt the behavior of bony fish.

Very little is known about the possible effects of ocean acidification on sharks, which like stingrays have skeletons made of cartilage. Scientists fear that ocean acidification could pose further problems for sharks, whose numbers are shrinking worldwide due to human activity.

To investigate the impact of ocean acidification on sharks, researchers experimented with 20 small-spotted catsharks (Scyliorhinus canicula), which live on the seafloor in temperate waters. The scientists exposed the sharks to atmospheric carbon dioxide levels that averaged either 401 or 993 parts per million for one month.

"The catsharks we used are very easy to work with because they are very sturdy and calm," said study co-author Fredrik Jutfelt, a marine biologist at the University of Gothenburg in Sweden. "They even come with a non-slip grippy sandpaper-like skin, which is a change from working on bony fishes."

The sharks exposed to high carbon dioxide levels appeared robust, with normal growth, mortality and metabolism. In addition, there was no damage seen to the tooth-like scales sharks possess known as dermal denticles.

"With long-term exposure to the acidic conditions, I would have expected to see some degradation occurring to them," said marine biologist Danielle Dixson at the Georgia Institute of Technology in Atlanta, who did not take part in this research. "It is really good that this was not shown."

However, sharks exposed to high carbon dioxide levels showed altered blood chemistry. They accumulated bicarbonate to keep their blood acid levels normal, much like how people with indigestion might take antacids to quell stomach acidity.

The researchers also filmed the swimming patterns of the sharks overnight using infrared cameras and infrared light the sharks could not see. While sharks exposed to current carbon dioxide levels had a nocturnal swimming pattern distinguished by many starts and stops, the investigators discovered that sharks exposed to high carbon dioxide levels had fewer but longer bouts of swimming, and swam to the surface more often.

"The results sort of build up over a long time," Jutfelt said. "The changes in behavior only became obvious after weeks of video analysis."

The scientists are uncertain whether the abnormal behavior they saw is due to stress from the sharks' constant searching for better water or from changes in blood chemistry affecting the brains of the sharks. In either case, these findings suggest ocean acidification could directly affect sharks.

"Previous findings have shown that ocean acidification will cause behavioral abnormalities in many species," Dixson said. "This study shows that the sharks examined here may suffer consequences through increased time spent continuously swimming." Dixson and her colleagues recently also found that acidic waters might rob sharks of the ability to smell food.

One key unanswered question of this study "is how well a month of carbon dioxide exposure represents decades of slowly increasing carbon dioxide levels," Jutfelt said. "There is unfortunately no way around this problem other than to wait and see."

"It would be very interesting if the sharks could be treated for a longer period of time to determine if the changes in blood plasma eventually level out or if the increased time spent continuously swimming does have a cost associated with it, such as reduced growth rate," Dixson said.

Jutfelt and his student Leon Green detailed their findings online Sept. 17 in the journal Biology Letters.

Charles Q. Choi is a freelance science writer based in New York City who has written for The New York Times, Scientific American, Wired, Science, Nature, and many other news outlets. He tweets at @cqchoi. Reprinted with permission from Inside Science, an editorially independent news product of the American Institute of Physics, a nonprofit organization dedicated to advancing, promoting and serving the physical sciences. Image: Fredrik Jutfelt and a catshark. Photo courtesy of Leon Green