Seasonal variation in the diel spawning time of the coral reef fish Oxymonacanthus longirostris (Monacanthidae): Parental control of progeny development

In the tropics, the longnose filefish Oxymonacanthus longirostris spawns shortly before sunset. In subtropical Okinawa Island, Japan, however, diel spawning time changed seasonally depending on seasonal changes in water temperature. At that site, spawning occurred in the morning during the early and late breeding seasons, when water temperatures were low. In the middle breeding season, when water temperatures were high, spawning occurred shortly before sunset. Hatching time of embryos was fixed, however, as embryos hatched just after sunset on the second day after spawning throughout the breeding season. Thus, the incubation period from fertilization of eggs to hatching of embryos changed seasonally and was negatively correlated with water temperature. The cumulative temperature from fertilization to hatching was almost constant throughout the breeding season, contributing to the control of larval development rather than the timing of hatching. Consequently, the developmental stage of newly hatched larvae was similar throughout the breeding season. In a rearing experiment that manipulated water temperatures, larvae which hatched at lower than natural ranges of cumulative temperatures were at an extremely immature developmental stage compared to larvae in natural temperature conditions. When water temperatures are low in the breeding season, O. longirostris parents shift their spawning time to the morning to ensure their embryos hatch as well-developed larvae. In conclusion, seasonal variation in the diel spawning time of subtropical O. longirostris assemblages will be a local adaptation to the strong seasonal changes of water temperature in their environment.