Short-term decreases in sea-water temperature can be more damaging to corals than short-term increases, although in the long term elevated temperatures are more harmful. The research, published in Scientific Reports, provides the foundation for a new framework to assess evolution and conservation of coral reefs. The increasing frequency and intensity of periods of extreme temperatures associated with global climate change are expected to have dramatic impacts on coral growth, bleaching and fitness but a comprehensive assessment of the health of corals and their endosymbiotic dinoflagellates when facing both hot and cold temperatures has been lacking. Melissa Roth and colleagues studied the effects of cold and heat stress on the physiological processes in reef-building corals. They subjected the branching coral Acropora yongei either to cold (21°C), hot (31°C) or control (26°C) temperatures in a laboratory setting and studied the effects over 20 days. Hot and cold temperature changes both negatively affected the corals but on different time scales, the authors report. During the first five days, cold-treated corals showed a greater decline in growth and increase in photosynthetic pressure than heat-treated corals. Ultimately, however, temperature increases were more stressful. These results provide new insights into the biology of corals, which could be important for the development of effective management strategies for the conservation of coral reefs on a global scale.