Spectacular tropical clownfish who live around ocean coral reefs rely heavily on their hearing, say scientists. It helps them find a mate, forage for food, and crucially avoid predators lurking in the depths.

Scientists from the University of Bristol in England bred baby clownfish then exposed different groups of them to different levels ofcarbon dioxide in the water around them -- one at today's levels, and the others at the levels the world's oceans are predicted to reach by 2050 and the year 2100.

They tested their hearing by piping into each tank the sounds of the coral reef -- those in today's conditions swam away from the noise of predators. Those in the mocked-up waters of the future showed no response, suggesting they couldn't hear.

Scientists say the more carbon dioxide we emit, the more oceans absorb and the more acidic the water becomes, and that has potentially devastating consequences for fish. What they don't know though is whether fish will be able to adapt and tolerate the changing waters.