Environment

Going deaf is not a problem that most of us would automatically associate with global warming. For coral reef fish, however, hotter seas could pose a real threat.

Young coral reef fish with misshapen ear bones are more likely to get lost and die, and exposure to warmer waters makes the problem worse, according to a study of fish living around Lizard Island on the Great Barrier Reef, Australia.

After hatching, most reef fish spend a few weeks out in the open ocean before returning to the reef to settle down. And it seems that sound is a key factor in guiding them to the right habitat.

The young fish have to home in on the high-frequency noises made by invertebrates like shrimp and sea urchins, and avoid the low-frequency noises made by crashing waves and adult fish.

Monica Gagliano at the Australian Institute of Marine Science in Townsville, Queensland, and colleagues found that at hatching, just over half of Ambon damselfish had asymmetrical otoliths, or ear bones.

Lost at sea

The team had suspected that it might be harder for these fish to pinpoint the origin of a sound, increasing the chance they would get lost in the ocean. And, indeed, their results showed that the asymmetrical fish were significantly less likely to make it back to the reef.

The team also broadcast high frequency and low frequency sounds from traps laid close to the reef, and found that the fish attracted to the high frequency traps - mimicking invertebrate food sources - were more likely to have symmetrical otoliths. (Listen to high frequency noises here, and low frequency sounds here.)

Gagliano says that as-yet-unpublished work shows that exposing adult reef fish to higher water temperatures and increasingly acid water - both of which are associated with global warming - increases the percentage of offspring born with asymmetrical otoliths.

Common problem

Increased acidification reduces the availability of calcium to be absorbed by fish to make bones. "And general stress, such as having to regulate their internal pH when it is changing in the water, also seems to affect the development of otoliths in the baby fish," says Gagliano.

The work suggests that global warming could have an impact on the number of fish returning to a reef, and so disrupt reef ecosystems, she says.

But while there's a correlation between otolith asymmetry and increased mortality, a direct cause hasn't been proven, says Arthur Popper, director of the Aquatic Bioacoustics Laboratory at the University of Maryland, College Park, US.

Otolith asymmetry is extremely common, Popper points out, and he says he's not convinced that it would affect a fish's ability to locate a sound. Other factors might explain the difference in proportions of fish making it back to the reef, he says.

Journal reference: Proceedings of the Royal Society B (DOI: 10.1098/rspb.2007.1388)

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