Spotty fish live in social groups or harems with one principal male but if he dies the dominant female will change sex and take over his role, Dr Muncaster says.
"Spotty fish are very territorial so if the male is taken out you can get this behavioural change that is stimulated from what's happening in the external environment. This leads to changes in the brain chemistry and shortly after, the most dominant female will no longer have a functional ovary but will begin to form a testis out of the same organ."
Dr Muncaster has run tests to establish how long the sex changing process takes. Blood samples, hormone levels and tissue were examined to determine timeframes.
"We looked under the microscope at tissue to follow the internal restructure of the ovary degenerating and beginning to build into a testis.
"You could see the process taking place where originally there was an ovary with eggs that became a testis that produced sperm."
The female had the ability to change into a fully functioning male in about eight weeks, he says.
Dr Muncaster's research focused on the internal, reproduction and cellular make-up of the spotty while the chemical reaction from its brain was being analysed in Dunedin with state of the art technological equipment.
The collaboration is ongoing but could have future benefits to aquaculture and medical science, he says.
"Global fisheries are declining and aquaculture is increasing and you have to be able to control reproduction to successfully breed in captivity, so we are using the spotty as a model species for studying sex change. Controlling sex ratios of broodstock fish is important in aquaculture."
Blue cod, breams, such as snapper and tropical groupers, are all commercially important sex changing fish, he says.
From a wider perspective body organ restructuring could be potentially important for medical science, Dr Muncaster says, and understanding this process in the spotty may provide insight for future organ manipulation.
"For a female to produce an egg and a male to produce sperm they come from stem cells. So the question is how does a stem cell switch to produce different germ cells and what tells it to do it," he says.
"The cellular machinery producing the hormones that support the development of eggs and sperm have to restructure so understanding that science could be useful to medical science."
