Ocean acidification alters the thermal performance curves of brooded larvae from the reef coral Pocillopora damicornis

Establishing the thermal reaction norm of coral larvae under elevated pCO₂ is crucial to anticipate how larval dispersal and population maintenance may be affected by future climate change. Here, we characterized the functional relationship between temperature (27−33 °C) and larval performance of the reef coral Pocillopora damicornis under two pCO₂ levels. The results showed that the temperature threshold of larvae was between 32 and 33 °C, as evidenced by the abrupt declines in photochemical efficiency and symbiont density, whereas no oxidative damage was observed between 27 and 33 °C and elevated pCO₂ did not influence any of these parameters. In addition, larval respiration and photosynthesis rates exhibited parabolic responses to temperature, and this relationship conformed to the Gaussian–Gompertz model with an optimal temperature around 31.5 °C, which was approximately 2.5 °C above the summer mean temperature, suggesting the potential for thermal acclimation. Most importantly, elevated pCO₂ significantly enhanced the larval photosynthesis and the stimulatory effect of elevated pCO₂ on the photosynthetic rates and capacity was more pronounced in cool and warm temperatures, indicative of shifted thermal sensitivity under high pCO₂. These results suggest that ocean acidification could alter the thermal performance curves and tolerance window of brooded P. damicornis larvae, with profound and important implications for larval ecology in a future changing ocean.