Effects of parental heat stress on the larval thermal performance of Porites astreoides, a brooding coral

In recent years, global coral cover has decreased due to the increased frequency and intensity of disturbance events. Increased seawater temperatures disrupt the symbiotic relationship between corals and photosynthetic algae, causing the bleaching events to occur worldwide. While many studies have described the impacts of bleaching on adult coral colonies, few have focused on the latent effects of temperature stress that occurs during larval brooding periods. Additionally, the plasticity of thermal performance by coral larvae remains poorly understood, including potential trans-generational transference of thermal tolerance traits. This study examines the metabolic performance of larvae from the common coral Porites astreoides (Yellow Mustard Hill coral) using a thermal performance curve approach to determine the impact of heat exposure during the brooding period on larval thermal performance. Adult colonies of P. astreoides were collected 3 weeks prior to expected planulation and maintained either under a thermal stress scenario (31°C) or ambient temperature conditions (28°C) in a flow through mesocosm facility. Larvae released from these colonies were then exposed to a range of temperatures from 17°C to 37°C and dark respiration was quantified as total oxygen consumption per mm³. If prior heat exposure resulted in trans-generational acclimatization then we would expect an upward shift in thermal performance by larvae brooded under thermal stress. However, no significant difference in thermal performance was found between treatment groups, with larvae from both conditions having thermal optimums of ~33°C and critical thermal maximums of ~42°C. Similar thermal performance curves despite differential brooding experiences suggests that parental history might not directly impact larval thermal tolerances. Moreover, the broad range of temperature tolerances and the high critical thermal maximum suggests that coral larvae are highly plastic in their response to temperature and may represent a critical life stage for acclimatization to future climate conditions.