Biological Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam.
J Exp Biol. 2011 Nov 1;214(Pt 21):3649-57. doi: 10.1242/jeb.059899.
Contemporary theory for thermal adaptation of ectothermic metazoans focuses on the maximization of energy gain and performance (locomotion and foraging). Little consideration is given to the selection for mechanisms that minimize resting energy loss in organisms whose energy gain is severely constrained. We tested a hypothetical framework for thermal performance of locomotor activity (a proxy for energy gain) and resting metabolism (a proxy for energy loss) in energetically compromised snails in the littoral fringe zone, comparing this with existing theory. In contrast to theory, the thermal ranges and optima for locomotor performance and metabolic performance of Echinolittorina malaccana are mismatched, and energy gain is only possible at relatively cool temperatures. To overcome thermal and temporal constraints on energy gain while experiencing high body temperatures (23-50°C), these snails depress resting metabolism between 35 and 46°C (thermally insensitive zone). The resulting bimodal relationship for metabolism against temperature contrasts with the unimodal or exponential relationships of most ectotherms. Elevation of metabolism above the breakpoint temperature for thermal insensitivity (46°C) coincides with the induction of a heat shock response, and has implications for energy expenditure and natural selection. Time-dependent mortality is initiated at this breakpoint temperature, suggesting a threshold above which the rate of energy demand exceeds the capacity for cellular energy generation (rate of ATP turnover). Mortality in a thermal range that elevates rather than limits aerobic metabolism contrasts with the hypothesis that cellular oxygen deficiency underlies temperature-related mortality. The findings of this study point to the need to incorporate aspects of resting metabolism and energy conservation into theories of thermal adaptation.
现代外温动物热适应理论主要集中在能量获取和性能（运动和觅食）的最大化上。对于那些能量获取受到严重限制的生物，其能量损失最小化的机制选择则很少被考虑。我们测试了一个关于运动活性（能量获取的代理）和静止代谢（能量损失的代理）的热性能的假设框架，比较了这个框架与现有理论。与理论相反，Echinolittorina malaccana 的运动表现和代谢表现的热范围和最佳温度不匹配，并且只有在相对较冷的温度下才能获得能量增益。为了克服在经历高体温（23-50°C）时对能量获取的热和时间限制，这些蜗牛在 35 到 46°C 之间（热不敏感区）降低静止代谢。代谢对温度的双模态关系与大多数外温动物的单峰或指数关系形成对比。代谢在热不敏感的断点温度以上的升高与热休克反应的诱导一致，这对能量消耗和自然选择有影响。在这个断点温度下开始出现依赖时间的死亡率，这表明能量需求的速率超过细胞能量产生的能力（ATP 周转率）的阈值。在提高而不是限制有氧代谢的热范围内的死亡率与细胞缺氧是导致与温度相关的死亡率的假说相反。本研究的发现表明，需要将静止代谢和能量节约的方面纳入热适应理论中。
