Shorter recruitment period and higher fecundity in high latitudinal populations of O. latipes species complex

In many organisms, environmental temperature is a key determinant of the reproductive season, although physiological thermal responses related to reproduction often vary among taxa. Comparative analyses of reproductive seasonality within and between species can provide insights into the physiological constraints and local adaptation of reproductive traits in wild populations. This study examined wild populations of medaka (Oryzias latipes species complex). Although the photo-thermal thresholds for gonadal maturation in this species have been studied extensively in laboratory settings, geographical patterns of seasonality in gonad maturation, fecundity, and recruitment have not been thoroughly investigated in wild populations. We examined seasonal changes in gonad weight and the proportions of juveniles in wild populations of medaka at different latitudes. Our findings showed that seasonal changes in gonad weight were more pronounced in a high-latitude population in Aomori compared to a low-latitude population in Okinawa, irrespective of sex. Annual average proportions of juveniles tended to be higher in the Okinawa population compared to the Aomori and Chiba populations, suggesting a shorter recruitment period in high-latitude populations. A comparison of body size and egg number indicated that fecundity was relatively higher in high-latitude populations. We discuss whether gonad maturation at high latitudes can be explained by seasonal changes in temperature. Interpopulation differences in the gonadosomatic index and fecundity support the adaptive evolution of increased reproductive investment in high-latitude environments with shorter reproductive seasons. Additionally, trade-offs between survival and reproduction may influence ecological longevity and annual recruitment dynamics in these populations.