Environmental temperature affects physiological functions, representing a barrier for the range expansions of ectothermic species. To understand the link between thermal physiology and biogeography, a key question is whether among-species thermal sensitivity of metabolic rates is mechanistically constrained or buffered through physiological remodeling over evolutionary time. The former conception, the Universal Temperature Dependence hypothesis, predicts similar among- and within-species thermal sensitivity. The latter conception, the Metabolic Cold Adaptation hypothesis, predicts lower among-species thermal sensitivity than within-species sensitivity. Previous studies that tested these hypotheses for fishes overwhelmingly investigated teleosts with elasmobranchs understudied. Here, we show that among-species thermal sensitivity of resting metabolic rates is lower than within-species sensitivity in teleosts but not in elasmobranchs. Further, species richness declines with latitude more rapidly in elasmobranchs than in teleosts. Metabolic Cold Adaptation exhibited by teleosts might underpin their high diversity at high latitudes, whereas the inflexible thermal sensitivity approximated by Universal Temperature Dependence of elasmobranchs might explain their low diversity at high latitudes.

环境温度影响生理功能，代表了变温物种范围扩张的障碍。要了解热生理学和生物地理学之间的联系，一个关键问题是代谢率的物种间热敏感性是否通过进化时间的生理重塑在机械上受到限制或缓冲。前一个概念，即普遍温度依赖性假说，预测物种间和物种内的热敏感性相似。后一个概念，即代谢冷适应假说，预测物种间的热敏感性低于物种内的敏感性。以前对鱼类的这些假设进行测试的研究绝大多数都调查了硬骨鱼类，而软骨鱼类研究不足。这里，我们表明，静息代谢率的物种间热敏感性低于硬骨鱼类的物种内敏感性，但在软骨鱼类中则不然。此外，与硬骨鱼类相比，软骨鱼类的物种丰富度随纬度下降的速度更快。硬骨鱼表现出的代谢冷适应可能是它们在高纬度地区的高度多样性的基础，而软骨鱼类的普遍温度依赖性近似的不灵活的热敏感性可能解释了它们在高纬度地区的低多样性。