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.海洋外温动物普遍对热胁迫较为敏感，部分生命阶段尤其脆弱，例如幼体或产卵亲体。本研究针对3种热带海洋鱼类——多棘刺雀鲷（Acanthochromis polyacanthus）、黑足小丑鱼（Amphiprion melanopus）与尖吻鲈（Lates calcarifer）的幼体及早期稚鱼阶段的临界热最大值（critical thermal maxima, CTmax）开展了系统调查。我们考察了发育驯化、生命阶段及实验升温速率的潜在影响，并测定了有氧代谢途径（柠檬酸合酶，citrate synthase, CS）与无氧代谢途径（乳酸脱氢酶，lactate dehydrogenase, LDH）相关的代谢酶活性。略高于常规的养殖温度既未对CTmax产生显著影响，也未改变CS活性——这一结果本可作为热驯化效应的佐证。但研究发现，不同物种及生命阶段的CTmax存在显著差异，该差异主要由体质量所解释；且升温速率越快，测得的CTmax越高。急性温度胁迫并未改变CS或LDH的活性，提示整体有氧与无氧代谢维持稳定。尖吻鲈为变态发育后从海洋生境洄迁至淡水生境的降海洄游物种，其CTmax显著高于另外两种珊瑚礁鱼类。本研究强调，若要获取鱼类物种高温上限的保守估计值，需对多个发育阶段与体质量范围进行检测。此外，高温上限的评估应采用标准化的升温速率。该方法不仅可为跨物种比较研究提供支撑，也有助于评估海洋鱼类的地理（重新）分布格局及气候变化敏感性。