Biannual coral spawning provides a mechanism for ecosystem resilience in Western Australia

Seasonal sea temperature variation plays a crucial role in marine invertebrate reproduction, with mass coral spawning dependent on specific climatic conditions. In Western Australia most corals reproduce in autumn when sea surface temperatures are at their peak, making them vulnerable to heat stress and reproductive failure. Our surveys of coral reproduction at Ningaloo Reef in 2022 and 2023 revealed spatial differences in reproductive patterns between the east and west coast of Ningaloo. A greater proportion of species spawn in the cooler spring season (30%) at a sheltered site on the east coast compared to on the west coast (4%). Meteorological and oceanographic data suggested that differences in wind and wave regimes on the east coast may allow for a wider range of spawning times. Furthermore, we hypothesise that higher temperatures on the east coast exert selective pressure on corals to spawn in the cooler spring to mitigate heat stress, which negatively impacts reproductive output, larval development, recruitment, and early coral survival. These findings suggest that divergent reproductive timing could foster local adaptation and create reproductive niches that may be a vital component of reef resilience in the coming decades.