Critical thermal maxima of early life stages of tropical fishes

Marine ectotherms are often sensitive to thermal stress, and certain life stages can be particularly vulnerable (e.g., larvae or spawners). In this study, we investigated the critical thermal maxima (CTmax) of larval and early juvenile life stages of three tropical marine fishes (Acanthochromis polyacanthus, Amphiprion melanopus, and Lates calcarifer). We tested for potential effects of developmental acclimation, life stage, and experimental heating rates, and we measured metabolic enzyme activities from aerobic (citrate synthase, CS) and anaerobic pathways (lactate dehydrogenase, LDH). A slightly elevated rearing temperature neither influenced CTmax nor CS activity, which otherwise could have indicated thermal acclimation. However, we found species- and life stage-specific differences in CTmax that were mainly explained by body mass, with faster heating rates leading to higher CTmax. Acute temperature stress did not change CS or LDH activities, suggesting that overall aerobic and anaerobic metabolism remained stable. Lates calcarifer, a catadromic species that migrates from oceanic to riverine habitats upon metamorphosis, had higher CTmax than the two coral reef fish species. We highlight that, for obtaining conservative estimates of a fish species' upper thermal limit, several developmental stages and body mass ranges should be examined. Moreover, upper thermal limits should be assessed using standardized heating rates. This will not only benefit comparative approaches but also aid in assessing geographic (re-) distributions and climate change sensitivity of marine fishes.