Ear may hold key to reef fish's sex change story

Melbourne, Nov 26 : The secret behind sex change occurring in reef fish lies in its ear, says an Australian marine scientist

Doctoral student Stefan Walker, of the School of Marine and Tropical Studies at James Cook University has found that the markings on the otolith, or ear stone, serves as rings on a tree in telling the age at which an individual fish changes sex.

Walker said that knowledge of when and how sex change occurs is important for aquaculture as it enables the industry to maximise fish reproduction.

He also said that in aquaculture the larger specimens, which are predominantly male, tend to be removed for consumption.

"We need to know when and how they change sex, so we know how males can be replenished to ensure reproduction is maximised," ABC Online quoted Walker as saying.

For the study, Walker induced sex change within a group of sharpnose sandperch, a common reef fish, where the females permanently defend an area and the males defend up to 10 females. All individuals mature first as females and later change sex to function as males.

Walker said that he induced sex change in the dominant female in the group by removing the male.

He compared growth of the sex-change female in the manipulated group with a dominant female in a control group after 30 days and found that the sex-change fish had undergone a period of rapid growth during its transition to a male.

It was also found that sex change was related to group size with the more females present in the group leading to a faster sex change in the dominant female.

However he also discovered that the metamorphosis into a male was captured through a series of ring-like markings that showed the daily otolith growth.

The otolith lies in a sac in the inner ear and as the fish moves it bounces around. It is critical in helping the fish determine spatial orientation and the detection of sound.

The fluctuation of the proportion of mineral to organic material used during otolith growth results in the production of a series of daily rings, which conveys the age of the individual just like in a tree. It was already known that the markings were formed as the fish moved from larval stage to its juvenile adult form. However, Walker found that the rings became thicker, darker and changed direction during sex change.

He said that not only do the ear markings show the sex change metamorphosis, it also "provides information on the magnitude of the maculinisation during sex change."

Walker said that the study shows the readjustment of sensory organs may be an important part of the sex inversion. As a male the fish is involved in more aggressive behaviour and physical combat associated with harem defence.

He said: "When the fish changes its sex it completely changes its way of life. It goes from being a sedentary female that hangs out in one spot, to a large male that is moving around trying to defend 10 females in one day.

"It isn't just an isolated event that changes the reproductive system. It is a synchronised and co-ordinated change in behaviour, morphology and sensory perception."

The study is published in the latest edition of the Royal Society's Biology Letters journal.

ANI