Ghosts of thermal past: reef fish exposed to historic high temperatures have heightened stress response to further stressors

Individual exposure to stressors can induce changes in physiological stress responses through modulation of the hypothalamic–pituitary–interrenal (HPI) axis. Despite theoretical predictions, little is known about how individuals will respond to unpredictable short-lived stressors, such as thermal events. We examine the primary neuroendocrine response of coral reef fish populations from the Îles Eparses rarely exposed to anthropogenic stress, but that experienced different thermal histories. Skunk anemonefish, Amphiprion akallopisos , showed different cortisol responses to a generic stressor between islands, but not along a latitudinal gradient. Those populations previously exposed to higher maximum temperatures showed greater responses of their HPI axis. Archive data reveal thermal stressor events occur every 1.92–6 yr, suggesting that modifications to the HPI axis could be adaptive. Our results highlight the potential for adaptation of the HPI axis in coral reef fish in response to a climate-induced thermal stressor.