Fish ears keep sex change record

You can tell when a fish has changed sex just by looking at its ears.

At least, Australian marine biologists Stefan Walker and Mark McCormick can - having just pioneered a new breakthrough for studying the behaviour and productivity of fish populations.

The researchers from the ARC Centre of Excellence for Coral Reef Studies and James Cook University have solved one of the major problems confronting fisheries biologists in determining the sustainability of fish populations - not knowing exactly when fish undergo a sex change.

"Many coral reef fishes - and other fish like barramundi - undergo a sex change at some point in their life - from male to female or female to male,"  Stefan explains. "This may be good breeding strategy for them, but it makes it very difficult for researchers to assess the productivity of the fish population if we don't know for sure when the sex change takes place."

With almost a third of world fisheries rated as having collapsed and many more under threat, and with coral reefs facing climate and other human-caused stresses, it is vital to assess the productivity of fish populations in order to know how much fishing pressure it can withstand and whether or not it can bounce back.  This includes having an understanding of the gender ratios and the age at maturity for females and males.

"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,"  he says.

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 may 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.

The researchers also have a theory that the larger and different shaped ear-stones in male perches may have something to do with the fine tuning of the fishes' spatial perception. Like humans, the inner ears of fish are not only receptive to sound, but are also receptive to individual movement and orientation. Sex changing fish may also change their otolith development so as to become more proficient in their new reproductive mode, which often involves moving around more complex terrain and engaging in physical combat with other males associated with competition for female partners. They intend to test this idea in subsequent research.

Their paper Fish ears are sensitive to sex-change is in the latest issue of Biology Letters of the Royal Society Journal.