The critical thermal maximum (CTMAX) is the temperature at which animals exhibit loss of motor response because of a temperature-induced collapse of vital physiological systems. A central mechanism hypothesised to underlie the CTMAX of water-breathing ectotherms is insufficient tissue oxygen supply for vital maintenance functions because of a temperature-induced collapse of the cardiorespiratory system. The CTMAX of species conforming to this hypothesis should decrease with declining water oxygen tension (PO2) because they have oxygen-dependent upper thermal limits. However, recent studies have identified a number of fishes and crustaceans with oxygen-independent upper thermal limits, their CTMAX unchanged in progressive aquatic hypoxia. The previous studies, which were performed separately on cold-water, temperate and tropical species, suggest the oxygen-dependence of upper thermal limits and the acute thermal sensitivity of the cardiorespiratory system increases with decreasing habitat temperature. Here we directly test this hypothesis by assessing the oxygen-dependence of CTMAX in the polar Antarctic krill (Euphausia superba), as well as the temperate Baltic prawn (Palaemon adspersus) and brown shrimp (Crangon crangon). We found that P. adspersus and C. crangon maintain CTMAX in progressive hypoxia down to 40 mmHg, and that only E. superba have oxygen-dependent upper thermal limits at normoxia. In E. superba, the observed decline in CTMAX with water PO2 is further supported by heart-rate measurements showing a plateauing, and subsequent decline and collapse of heart performance at CTMAX. Our results support the hypothesis that the oxygen-dependence of upper thermal limits in water-breathing ectotherms and the acute thermal sensitivity of their cardiorespiratory system increases with decreasing habitat temperature.

中文翻译：

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来自不同热栖息地的甲壳类动物热上限的氧依赖性

临界热最大值 (CTMAX) 是动物因温度引起的重要生理系统崩溃而表现出运动反应丧失的温度。假设水呼吸变温动物 CTMAX 的核心机制是由于温度引起的心肺系统崩溃，组织供氧不足，无法维持重要的维持功能。符合这一假设的物种的 CTMAX 应该随着水氧张力 (PO2) 的下降而降低，因为它们具有依赖于氧的热上限。然而，最近的研究已经确定了一些鱼类和甲壳类动物具有不依赖氧气的热上限，它们的 CTMAX 在进行性水生缺氧时保持不变。之前的研究分别针对冷水、温带和热带物种进行，表明热上限的氧气依赖性和心肺系统的急性热敏感性随着栖息地温度的降低而增加。在这里，我们通过评估极地南极磷虾 (Euphausia superba) 以及温带波罗的海虾 (Palaemon adspersus) 和褐虾 (Crangon crangon) 中 CTMAX 的氧气依赖性来直接检验这一假设。我们发现 P. adspersus 和 C. crangon 在低至 40 mmHg 的进行性缺氧条件下保持 CTMAX，并且只有 E. superba 在常氧下具有依赖于氧的热上限。在 E. superba 中，观察到的 CTMAX 与水 PO2 的下降得到了心率测量结果的进一步支持，该测量结果显示了在 CTMAX 下的平稳以及随后的心脏性能下降和崩溃。