Variations of light and temperature regimes and resulting effects on reproductive parameters in medaka (Oryzias latipes)

In seasonally breeding fish species, altered fecundity, fertility, and spawning interval are associated with changes in environmental cues such as temperature and photoperiod. To determine quantitative impact of these cues on a suite of reproductive endpoints, groups of medaka (Oryzias latipes, two breeding pairs per group) were subjected to varying photoperiod and temperature regimes. Embryo production ceased after photoperiod reduction from 16L:8D to 8L:16D (at 25°C). A severe decline in production was observed after a temperature decrease of 10°C (25°C to 15°C [16L:8D]). Under reduced photoperiod, histologic analysis showed no mature ova and moderate oocyte atresia in all individuals. However, reduced temperature (15°C) produced only mild oocyte atresia and fewer mature ova. Under both reduced photoperiod and reduced temperature regimes, mature spermatozoa were observed. Offspring viability, along with spawning interval, were not affected by photoperiod reduction. Temperature change had no effect on offspring viability but caused an increase in spawning interval. A shortened photoperiod profoundly affected medaka reproduction, whereas decreased temperature reduced, but did not arrest, fertility; reduced photoperiod decreased fecundity. These findings have important implications for culture of medaka as well as use of this teleost model for reproductive toxicology studies.