Tropical coral reefs are predicted to be amongst those most impacted by climate change since they have evolved under relatively thermally stable conditions. Furthermore, it is suggested that many species may already be living close to their upper thermal limits, with little capacity to acclimate to further warming conditions. Current maximum summer temperatures for the majority of coral reefs globally is 32C, with an additional 3-4C heating projected by 2100. However, the hottest coral reefs on Earth (Arabian Gulf) are already experiencing summer maxima above 35oC and an annual thermal range >17oC, creating a present day example of ocean warming impacts. Here, we use the Arabian Gulf reefs as a natural laboratory for climate change to investigate how three species of coral reef fishes (Scolopsis ghanam, Cheilodipterus novemstriatus and Escenius pulcher) have managed to acclimate and/or adapt to extreme thermal conditions. For each species we compared the metabolic performance (maximum metabolic rate, resting metabolic rate, aerobic scope and excess post-exercise oxygen consumption (EPOC)) across populations from the southern Arabian Gulf, and populations from the thermally more benign reefs in Gulf of Oman (annual temperature fluctuations 22-32C), focusing on five temperatures incorporating the existing seasonal Arabian Gulf thermal range (18, 22, 27, 31.5, 35.5oC). Fishes were caught throughout the year when SST matched that of experimental temperature, allowing an accurate representation of any naturally occurring acclimation. Additionally, as conditions in the Gulf of Oman do not reach the same seasonal extremes, fishes from this region were acclimated for >3 weeks to 18C and 35.5C before experimental procedure. Maximum and resting oxygen consumption rates were then measured for each species and population at each temperature, using static intermittent respirometry. Our results reveal that all three species from the Arabian Gulf displayed a shift in peak aerobic scope to higher temperatures than Gulf of Oman fishes, suggesting thermal adaptation has occurred in order to meet metabolic demands at higher temperatures. However, all three species showed a significantly reduced aerobic scope at both the coolest and warmest temperatures, leaving little additional energy available for enhancing ecological activities. Resting metabolic rate increased exponentially from 18oC to 35.5oC in all species and populations, indicating the importance of maintaining performance to acquire food and fulfill energetic demand. By using Arabian Gulf reef fish populations as a natural proxy for climate change, this study indicates that although there may be capacity for some reef fish species to adapt to survive projected increases in SSTs and thermal fluctuations, this comes at the cost of severely reduced aerobic performance at both ends of the scale.