Steroid hormones dynamics during coral reproduction: Multi-year patterns in Acropora eurystoma from the Red Sea

Sex steroid hormones have been identified in coral tissues and associated primarily with spawning events, yet successful spawning relies on the precisely timed gametogenesis. We investigated steroid hormone dynamics during gametogenesis in Acropora eurystoma from the Gulf of Aqaba over three reproductive cycles (2021-2023). Estrogen peaked in March, declined toward May, and rose by spawning (June-July). This pattern was inversely correlated with oocyte diameter, suggesting association between estrogen and gametogenic stage. Progesterone remained stable during reproductive months but peaked in October, potentially signaling initiation of the next reproductive cycle. Photoperiod and UV radiation were stronger predictors of estrogen levels than temperature, suggesting light-related cues may influence coral endocrine regulation. Despite uniform estrogen distribution, oocyte prevalence was higher in central than peripheral regions, indicating polyp-level modulation of colony-wide hormonal signals. These findings suggest hormonal profiling as an early-warning system for detecting reproductive disruption, with implications for coral conservation and restoration.