Marine heatwaves and chronic warming induce differing impacts to populations in a coral reef fish

Exposure to acute versus chronic warming can produce differing plastic responses. Although chronic warming during sensitive developmental stages has been shown to produce beneficial plasticity, the long-term impacts of acute marine heatwaves (MHWs) are poorly understood. Further, responses to warming can vary across species’ range as populations often possess different thermal physiology and plastic capacity. Here, we tested whether a developmental MHW (+1.5°C for 30 days after hatching) and chronic warming throughout development (+1.5°C from hatching) differentially affected low- and high-latitude populations of the coral reef damselfish Acanthochromis polyacanthus. At one year, we measured growth (standard length, mass, and mass for a given standard length [condition]), aerobic metabolism (resting metabolic rate [RMR], maximum metabolic rate [MMR], factorial aerobic scope [FAS], and absolute aerobic scope [AAS]), and immune response (swelling to phytohaemagglutinin) at three test temperatures (28.5°C, 30.0°C, and 31.5°C). We found co-gradient variation in thermal responses, with low-latitude fish having higher MMR and AAS across all test temperatures. In contrast, a developmental MHW only affected high-latitude fish, increasing condition but reducing aerobic performance (higher RMR and lower FAS). Similarly, chronic warming increased phytohaemagglutinin-induced swelling only in high-latitude fish, highlighting that high-latitude fish may experience greater phenotypic shifts in response to developmental warming. Importantly, however, chronic warming reduced standard length, mass, MMR and AAS in both latitudes. These findings demonstrate that acute and chronic warming may affect populations differently, with chronic warming potentially producing more widespread negative impacts than MHWs. Such intraspecific variation may help refine our understanding on how climate-change impacts may manifest across a species’ range.