To survive the complex, often-dangerous environment of a coral reef, the colorful reef fish Neopomacentrus azysron has to be a clever fish. Like many intelligent animals, it uses the right and left hemispheres of its brain for different purposes, which allows for quick problem-solving. But this reef fish could be in danger of losing its smarts as levels of CO2 in the ocean continue to rise due to human activity, according to a new study. Researchers raised one set of reef fish larvae in normal seawater and one set in water containing twice as much CO2—the level that the Pacific Ocean is expected to reach by 2100. When the fish grew up, the team put them in a maze. Each fish that had been spawned in the normal water consistently preferred to turn either right or left every time it reached the barrier, a sign of "handedness." But fish that had been spawned in high CO2 didn't have a favorite hand: they turned right or left at random  each time they hit the barrier. This loss of handedness, the researchers report today in Biology Letters, may be a sign of other, more subtle developmental brain dysfunctions that might hurt the ability of this fish, and other marine species that could be similarly affected by CO2, to survive in a high-carbon future.
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