Abstract
Introduction Thermal stress experiments on corals from habitats with contrasting environmental regimes in understudied regions, such as South Africa, are essential to elucidate the mechanisms underlying coral resilience/susceptibility to thermal stress. This study examined the thermal resilience/susceptibility of Anomastraea irregularis (massive morphology) and Pocillopora verrucosa (branching morphology) collected from environmentally variable intertidal and more stable subtidal habitats from the east coast of South Africa. Methods Corals were maintained in closed recirculating aquaria and exposed to two thermal stress conditions (28°C and 30°C) for three months, followed by recovery at control conditions (26°C) for two months. Rates of respiration, photosynthesis, and growth were measured monthly. The Symbiodiniaceae density, chlorophyll-a concentration, and lipid concentration were analysed at the start and end of the thermal stress and at the end of the recovery period. Results The treatments induced bleaching in both species from both habitats with an associated decrease in photosynthetic and growth rates. Inherent differences across all measured physiological parameters, including variation in survival, were observed between the species and habitat throughout the experiment, highlighting that environmental origin and possibly morphology can influence thermal tolerance. The intertidal corals were more tolerant than their subtidal conspecifics, and A. irregularis appeared more tolerant than P. verrucosa in both treatments. The intertidal corals decreased their respiration rates to control levels by the end of thermal stress and, both initially and throughout the study, maintained higher Symbiodiniaceae densities, chlorophyll-a, and lipid concentrations than their subtidal conspecifics. Discussion The tolerance of A. irregularis may stem from thicker tissue which allowed higher Symbiodiniaceae density and lipid concentrations and lower P:R ratios consistent with a more heterotrophic nature. The photosynthetic and growth rates, Symbiodiniaceae density, chlorophyll-a concentration, and lipid concentration of both species from both habitats did not recover two months after thermal stress indicating that restoration of physiological homeostasis following prolonged thermal stress likely requires substantially longer periods. Therefore, the results in this study should be interpreted as evidence of differential tolerance and early recovery responses rather than complete resilience. These findings provide insight into the physiological mechanisms underpinning thermal tolerance in high-latitude corals, highlighting that corals persisting in environmentally variable environments can exhibit traits critical for understanding resilience to climate change. An important caveat to consider, despite regular water changes, the closed recirculating systems may accumulate metabolic waste products over time, which can independently affect coral physiology and confound thermal stress responses.