Environmental drivers of acroporid coral reproduction: insights from gametogenic cycles on the shallow reef flats of Lankayan Island, Sabah, Malaysia

The timing of coral reproduction results from an interplay of ultimate and proximate factors, fostering increased synchrony from gamete production to spawning. Coral spawning and spawn slicks have been observed since 2012 at Lankayan Island, yet the reproductive characteristics of corals from this locality remain unknown. This study investigates the reproductive biology of acroporid corals at Lankayan Island and examines the environmental drivers influencing their gametogenic cycles. Monthly sampling of tagged colonies from November 2018 to November 2019 was conducted for 5 species ( Acropora florida , A. hyacinthus , A. pulchra , A. millepora , and Montipora digitata ). All species are hermaphroditic broadcast spawners. Acropora spp. exhibited annual gametogenesis with a single spawning season, while M. digitata displayed biannual gametogenesis with 2 spawning seasons. Regression analyses revealed that oocyte growth (changes in oocyte diameter; µm mo -1 ) of Acropora spp. were best explained by photoperiod and time of sunset, suggesting that seasonal light cycles may act as key proximate cues for gametogenesis. In contrast, none of the tested environmental factors had a significant association with oocyte growth in M. digitata , indicating possible reliance on different reproductive cues. This study provides critical baseline data for corals in Lankayan Island and contributes to a broader understanding of how environmental factors shape reproductive synchrony in low-latitude reefs. These insights are essential for predicting potential shifts in reproductive cycles under changing climate conditions and for informing conservation strategies in equatorial coral reef ecosystems.