Temporal dynamics of courtship and spawning in medaka under laboratory conditions revealed by 24 h video monitoring.

Understanding the biological phenomena in model organisms typically relies on laboratory studies. However, the ecological validity of these findings is often uncertain when natural behaviors remain understudied. The medaka (Oryzias latipes) is a widely used model in reproductive and behavioral research; however, the timing of spawning in natural settings has rarely been directly observed. Recent fieldwork has suggested that medaka spawn several hours before sunrise, in contrast to the common assumption in laboratory-based studies that spawning occurs within an hour before or after exposure to light. In this study, we conducted continuous 24 h video recordings of medaka pairs under controlled laboratory conditions (14 L:10D photoperiod) to quantify diel variations in courtship and spawning behavior. Spawning occurred mostly between 08:00 and 11:00, peaking immediately after lights were turned on (08:00). Courtship behavior began during the dark period, increased before lights-on, and peaked between 07:00 and 09:00. These results exhibited a consistent delay of 3-4 h compared to (semi-)natural conditions. Our results highlight the influence of environmental conditions on reproductive timing in medaka. The temporal shifts observed in this study underscore the importance of incorporating natural temporal dynamics into experimental protocols to ensure accurate behavioral studies with model organisms.