Circadian oviposition rhythm and locomotor activity in the medaka,Oryzias latipes

The time of oviposition and the locomotor activity rhythm of pairs of male and female medaka, Oryzias latipes, under LD 16:8 and continuous light were investigated. The fish layed eggs just before or after the onset of light periods in LD 16:8 and the time of oviposition gradually delayed every day when the fish were transferred from LD 16:8 to continuous light. Thus, the oviposition rhythm is clearly entrained by light‐dark cycles and it shows a circadian rhythm in continuous light. Phase‐angle differences of the oviposition rhythm were compared between two different intensities (0 and 5 lx) during dark periods to examine the masking effect of complete darkness, but there was no statistically significant difference. Four types of the locomotor activity rhythm were observed under LD 16:8; diurnal, nocturnal, light‐change active and arrhythmic. It seems to depend on the light intensity during dark periods whether the fish show diurnal or nocturnal behaviour. Only one out of 19 pairs showed a freerunning circadian rhythm when the fish were transferred from LD 16:8 to continuous light.