The scientists from the James Cook University and the Australian Institute of Marine Science found that fish with asymmetrical ear bones struggle to return to their home reef.

"In our opinion, ear bone asymmetry in the early life stages of reef fish interferes with their capacity to find and settle on coral reefs," fish ecologist Monica Gagliano said in a statement.

Fish at the end of their "ocean stage" after hatching navigate by homing-in on reef-associated sounds, such as the gurgling of fish and the snapping of crustaceans, said the scientists, whose study was published on Friday in the British scientific journal Proceedings of the Royal Society.

Vertebrate animals make sense of sounds by comparing differences in the acoustic signal between their two ears. To do this well, ear structures must be relatively symmetrical. Asymmetrical ear bones do not appear to make the fish deaf, but might interfere with the ability of the fish to hear effectively.

The scientists said ear bone asymmetry could be closely linked to rising sea surface temperature and acidity, caused by high atmospheric carbon dioxide levels, as well as localised stresses. Oceans absorb CO2 from the atmosphere, acting as a giant sink for the greenhouse gas.

Fish ear bones, like fish skeletons and reef-building corals, are made from calcium carbonate. When seawater becomes more acidic, there is less calcium carbonate available for building calcium-based structures, including fish ear bones.

LOST AT SEA

The scientists studied damselfish, which are abundant on Australia's Great Barrier Reef and western Australia's Ningaloo Reef, and found that at hatching, 41 percent of a sample group of fish had symmetrical ear bones and 59 percent asymmetrical.

When the scientists examined the ear bones of fish returning from open ocean to settle on the reef a few weeks later, far fewer asymmetrical fish made their way back to the reef.

The scientists also found that those with asymmetrical ear bones that did make it to the reef took longer to do so than their symmetrical counterparts.

"There is a degree of asymmetry that is acceptable in the population, some is natural," said scientist Martial Depczynski.

"Not all the babies are created equal and not all of them are going to make it, even in pristine environments," said Depczynski.

But Depczynski said the already high mortality rate among reef fish hatchlings was likely to rise even higher if young fish could not navigate by sound.

The scientists said they suspected asymmetrical ear development might be responsbile for a drop in the number of damselfish in recent years, but more study was needed.

"Five years ago we used to see them in the thousands, now they are not so plentiful," said Gagliano.

(Editing by David Fogarty)