Effects of temperature, hyposalinity, and diminishing sperm concentration on fertilisation and embryonic development in Acropora tumida and Platygyra carnosa.

Evaluating climate change impacts, for example, temperature anomaly and hyposalinity, on coral reproduction across a diversity of systems, including high-latitude marginal environments, is important. Here, we examined temperature and hyposalinity impacts on fertilisation under various sperm concentrations, and embryo development of two locally common hard corals, Acropora tumida and Platygyra carnosa, in Hong Kong. A salinity of 22 psu reduced fertilisation success by ~ 83% for both species. Our results demonstrated that increasing sperm concentration will be required to achieve the same amount of fertilisation success (i.e. 50%) under decreasing salinity. Temperature had species-specific effects on fertilisation. Decreased fertilisation was recorded under both lowered (– 3 °C) and elevated temperature (+ 3 °C) for A. tumida. For P. carnosa, fertilisation decreased under lowered temperature but increased under elevated temperature. For embryonic development, 26 psu salinity resulted in 33 and 40% more abnormal embryos for A. tumida and P. carnosa. The effects of temperature on embryonic development again varied between species. Under ambient salinity, elevated temperature led to 60% more abnormal embryos for P. carnosa, but only 8% for A. tumida. With reef degradation, diminishing sperm abundance will interact with rising climate change stressors and potentially impair coral reproduction.