Global warming leads to a significant decrease in body size of aquatic ectotherms, as shown by a meta-analysis of long-term data, experimental studies, and published results. This finding supports three ecological rules: Bergmann's rule, James' rule, and the Temperature-Size Rule (TSR). The study reveals that warming causes a shift toward smaller species and younger age classes, reducing size-at-age. These changes are observed across various aquatic organisms, including fish, plankton, and bacteria. The results indicate that reduced body size is a third universal ecological response to global warming, alongside shifts in species ranges and seasonal life cycle events. The study also highlights the importance of considering temperature as a key factor in understanding ecological changes, as it influences body size at individual, population, and community levels. While fisheries may contribute to size reduction in some species, the primary driver appears to be warming. The findings emphasize the need for further research to understand the mechanisms linking temperature and size changes, and to apply ecological theories like the Metabolic Theory of Ecology to explain these patterns. Overall, the study underscores the critical role of temperature in shaping aquatic ecosystems and the importance of considering thermal constraints when assessing the impacts of climate change on biological systems.Global warming leads to a significant decrease in body size of aquatic ectotherms, as shown by a meta-analysis of long-term data, experimental studies, and published results. This finding supports three ecological rules: Bergmann's rule, James' rule, and the Temperature-Size Rule (TSR). The study reveals that warming causes a shift toward smaller species and younger age classes, reducing size-at-age. These changes are observed across various aquatic organisms, including fish, plankton, and bacteria. The results indicate that reduced body size is a third universal ecological response to global warming, alongside shifts in species ranges and seasonal life cycle events. The study also highlights the importance of considering temperature as a key factor in understanding ecological changes, as it influences body size at individual, population, and community levels. While fisheries may contribute to size reduction in some species, the primary driver appears to be warming. The findings emphasize the need for further research to understand the mechanisms linking temperature and size changes, and to apply ecological theories like the Metabolic Theory of Ecology to explain these patterns. Overall, the study underscores the critical role of temperature in shaping aquatic ecosystems and the importance of considering thermal constraints when assessing the impacts of climate change on biological systems.