Parapercis cylindrica a species of reef perch used in the study. Researchers found that its ear bones change in growth when it undergoes a sex change from female to male.
Credit: ARC COE
SYDNEY: Many reef fish undergo a sudden sex change during their lives and understanding when is important for measuring the health of fisheries. Now researchers reveal that you can tell when the gender bending occurs just by looking at their ears.
"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," said Stefan Walker of the of the ARC Centre of Excellence for Coral Reef Studies and James Cook University in Townsville, Queensland.
Sometimes this is because the fish grow throughout their lives, and a larger fish makes a more successful job of breeding as one sex or the other.
Good breeding strategy
"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, or what factors influence its occurrence," he said
With almost a third of world fisheries having collapsed and many more under threat, it's important to understand the sustainability of populations in order to know how much fishing pressure they can withstand, and whether or not they can bounce back. This includes understanding gender ratios and the age at maturity for females and males.
"Unfortunately in fish 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
To learn more, he and co-worker Mark McCormick decided to look at fishes' ear stones, or otoliths; tiny bones which develop through the deposition of daily layers. Like tree rings, these give an age-based history of the individual's growth.
Harems and ear growth
By examining a small species of reef perch (Parapercis cylindrica), the experts found that sex change effects otolith growth, giving an "age-specific, sex-change signature." This consisted of a dense region in the otolith which corresponded with the time when their subject fish changed from female to male.
Furthermore, they discovered, that 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 said.
The Australian study is to be published in an upcoming issue of the journal Biology Letters.
