Temporal shifts in ovulation between laboratory and semi-natural environments in the model fish medaka

Abstract Research on model organisms in controlled laboratory settings has enabled fundamental biological discoveries. However, the ecological validity of these findings remains uncertain when natural ecology is poorly understood. Medaka (Oryzias latipes) has served as a model fish for over a century. Recent studies on medaka have shown that the timing of daily mating behaviour differs by 3–4 h between laboratory and natural conditions; however, the underlying physiological timing of reproduction remains unclear. This study compared daily ovulation timing in medaka that spawn under standard laboratory conditions (artificial photoperiod, constant water temperature) and semi-natural conditions (natural light, daily temperature fluctuations) by dissecting ovaries sampled hourly to assess ovulation status. The time when 50% of individuals had ovulated was 4.2 h before sunrise in semi-natural conditions and 0.7 h before lights-on in laboratory conditions, indicating a 3.5 h (2.6–4.3 h) shift. These quantitative results revealed the significant physiological differences between laboratory and natural conditions despite the controlled conditions typical of laboratory reproductive studies. These findings have important implications for studies on circadian rhythms and reproductive physiology. They also emphasize the necessity of incorporating ecological context into an experimental design to ensure the biological relevance and reproducibility of model organism research.