Principles of Genetics and Breeding

4.1.8 Metagamic sex determination

Although sex determination is primarily under genetic control, environmental factors such as temperature, photoperiod, salinity and even crowding can help to determine sex in fish.

• Environmental studies of sex ratios have provided evidence for a non-genetic (metagamic) sex determination mechanism in European and Japanese eels (Anguilla japonica).
• Nutritional or other effects of crowding and competition induced development into males, which mature at a much smaller size than females.
• There is a possibility that pH of water during reproduction may influence sex ratio.
• In Pelvicachromis and Apistogramma, sex ratios are strongly biased toward females in basic water and toward males in acid water.
• Both activity and longevity of sperm are affected by pH and it is possible that the pH of the buccal cavity may be adjusted by special glands in ova incubators.
• Accordingly, population density is the most probable factor in sex determination.
• Potential males may develop into females if they are solitary.