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Increasing temperatures are expected to significantly affect the physiological performance of ectotherms, particularly in tropical locations. The shape of an organism's thermal reaction norm can provide important information on its capacity to persist under climate change scenarios; however, difficulty lies in choosing a measurable trait that best depicts physiological performance. This study investigated the effects of elevated temperatures on processes related to oxygen uptake and delivery, including oxygen consumption, haematology, and tissue health for a low-latitude population of coral reef damselfish. Acanthochromis polyacanthus were collected from the Torres Strait (10 degrees 31-46'S, 142 degrees 20-35'E) and maintained at current average ocean temperatures (+0 C; seasonally cycling), + 1.5 degrees C and + 3 degrees C higher than present day temperatures for 10 months. Aerobic performance indicated a limit to metabolic function at + 3 degrees C (33 degrees C), following an increase in aerobic capacity at + 1.5 degrees C (31.5 degrees C). Neither haematological parameters nor gill morphology showed the same improvement in performance at + 1.5 degrees C. Gill histopathology provided the first indicator of a decline in organism health, which corresponded with mortality observations from previous research. Findings from this study suggest thermal specialisation in this low-latitude population as well as variation in thermal sensitivity, depending on the physiological trait.