Clownfish, the colourful tropical species that stars in the movie Finding Nemo, appear to lose their hearing in water slightly more acidic than normal.
The fish did not respond to the sounds of predators at levels of acidity that may be common by the end of the century, the BBC reports.
Scientists report in the online version of the journal Biology Letters that failing to move away from danger would hurt the fish's survival.
The oceans are becoming more acidic because they absorb much of the carbon dioxide that humanity puts into the atmosphere.
The researchers raised baby clownfish in tanks containing water at different levels of acidity.
One resembled the seawater of today, with the atmosphere containing about 390 parts per million (ppm) of carbon dioxide. The other tanks were set at levels that could be reached later this century - 600, 700 and 900 ppm, the BBC reports.
The more carbon dioxide there is in the atmosphere, the more the oceans absorb - and the more they absorb, the more acidic the water becomes.
In the experiment, the fish could decide whether to swim towards or away from an underwater loudspeaker replaying the sounds of predators recorded on a reef.
In water with today's levels of carbon dioxide, the fish spent three-quarters of the time at the opposite end of the tube from the loudspeaker.
But at higher concentrations, the clownfish showed no preference, suggesting they could not hear, could not decipher, or did not act on the warning signals.
Previous research has shown that fish also lose their capacity to scent danger in slightly more acidic seawater.
