The effect of operational sex ratio on fertilization success and clutch size in Japanese medaka (Oryzias latipes)

Abstract

The operational sex ratio is the number of fertilizable females to mature males in a population at a particular time. Variation in this ratio is often associated with change in behaviour during mating, including differences in male tactics. In Japanese medaka (Oryzias latipes), a species of freshwater fish found throughout Japan, there are two types of male alternative mating tactics: sneaking in which small males attempt to achieve some fertilization success by joining a spawning pair and releasing sperm, and interference where there is a disruption to a reproductive event. In addition, females adjust their clutch sizes in response to male behaviour. The operational sex ratio in this species varies across latitudes and this variation is linked to differences in mating behaviour and morphology. The objective of this work is to determine whether clutch size and proportion of fertilized eggs varies with differing operational sex ratio. To measure these responses, I collected eggs from Japanese medaka under four experimental operational sex ratios. I determined fertilization success and clutch size for females, as well as female growth rate over a 4-month period. There was no significant difference in fertilized eggs or clutch size among all four treatments. This can be attributed to fertilization assurance in higher operational sex ratios.