Sydney, Dec 2 (IANS) Coral reef fishes are experts at changing sex, becoming either male or female — but when, researchers didn’t have any clues.However, a breakthrough pioneered by marine biologists Stefan Walker and Mark McCormick of the ARC Centre of Excellence for Coral Reef Studies and James Cook University, has identified their ears as recording this change.

Their finding has solved major problems confronting fisheries biologists in determining the sustainability of fish populations - not knowing exactly when fish switch sex.

With almost a third of world fisheries rated as having collapsed and many more under threat, it is vital to understand the gender ratios and the age at maturity for females and males, according to a James Cook University release.

“Unfortunately, in fish that change sex, this is hard to get a handle on because the change can happen at different times. We needed a tool that would tell with accuracy when sex change has taken place or is likely to occur,” said Walker.

The team decided to focus on the fishes’ ear stones, or otoliths, which develop through the deposition of daily layers, providing an age-based history of the individual’s growth.

They proposed that the process of sex change might effect otolith growth, resulting in formation of an age-specific sex-change signature.

To their delight they found a dense region in the otolith material that corresponded exactly with the time when their subject fish - a small reef perch - changed from female to male.

Furthermore as soon as the new males acquired a harem of females, their ear-stones began to grow much more rapidly and in a different direction than when they were females.

And the more females they had, the faster and larger their ‘ears’ grew.

This new information about sex change and otolith development can help fisheries scientists to more accurately assess the dynamics and productivity of hermaphroditic stocks, the researchers say.

These findings have been published in the latest issue of Biology Letters of the Royal Society Journal.