Abstract
The social condition of bi‐directional sex change in the gobiid fish Trimma okinawae was investigated at Akamizu Beach, Kagoshima, Japan. Social groups of T. okinawae usually consisted of a large male and one or more smaller females. The number of females in the group was positively correlated with male body size and groups were usually separated from each other by 1–3 m. In total, 22 instances of female‐to‐male sex change and three instances of male‐to‐female sex change were observed during the 16 months that social groups were monitored. Two individuals changed sex twice: female to male and back to female. Female‐to‐male sex change occurred when the male disappeared from a group. Either the largest remaining female changed sex to male or a large female from another group immigrated and changed sex to male. Larger individuals appear to benefit from becoming male because they can monopolize the breeding opportunities with several females, as reported in other protogynous fishes. Sex change from male‐to‐female only occurred when a solitary male joined another group as a subordinate. Mortality rates are high in these small fish, therefore joining another group and reproducing as a female is likely to increase the reproductive value of a solitary male.