Changes in the chemistry of the world's oceans expected by century's end could impact the hunting ability of sharks, which depend heavily on their sense of smell to locate prey, researchers say.

As ocean waters turn increasingly acidic from absorbing atmospheric CO2 created by human activities, the odor-detecting ability of sharks to locate prey could diminish, they say.

In an experiment, scientists placed 24 sharks taken from the waters in the region of Cape Cod in a 30-foot pool with two lanes of seawater running into it.

Into one lane, the researchers pumped the odor of a squid, while the other lane was kept flowing with normal seawater.

As expected, the sharks spent more time below the lane containing squid odor, the researchers said.

However, when the scientists altered the acidity of the water, the sharks spent much less time around the squid lane, and in fact some actively avoided it, the scientists reported in the journal Global Change Biology.

"The sharks' tracking behavior and attacking behavior were significantly reduced," says biologist Danielle Dixson of the Georgia Institute of Technology. "Sharks are like swimming noses, so chemical cues are really important for them in terms of finding food."

The study was the first to test sharks' capability of sensing and locating the odors emitted by their prey under conditions simulating acidity levels predicted for the world's oceans by the end of the century, the researchers said.

The carbon dioxide concentrations introduced in the experiment are consistent with most climate change predictions from the middle of the century up to 2100, they said.

Acidic waters impact fish behavior because they disrupt a specific neural receptor present in most marine organisms possessing a nervous system. When the receptor, known as GABAA, ceases to work, neurons no longer fire properly.

Sharks are ancient creatures, and have no doubt adapted in the past to changing ocean acidification conditions, but have never faced such changes occurring as rapidly as they are today, the researchers said.

"It's the rate of change that's happening that's concerning," Dixson says. "Sharks have never had to deal with it this fast."

The effect of acidification on the feeding behaviors so vital to the survival of already threatened global shark populations could have cascading effects down through the marine ecosystem, scientists say.

And what affects predators also affects prey; previous studies have shown that fish living in and around coral reefs where water is becoming more acidic are less capable of detecting the odor of approaching predator fish.

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