Pacific bluefin tuna should be hit with cardiac arrest every time they dive deep. Though their bodies remain warm, their hearts receive blood directly from the gills, which has a temperature similar to the surrounding water. Such a quick temperature change would stop the hearts of most other animals, including humans. Now, scientists have figured out how the fish survive. They report this month in the Proceedings of the Royal Society B that changes in the heartbeat due to temperature (the heart slows as it cools and then speeds back up as the fish resurfaces) and adrenaline, released from the stress of diving, alter the electrical activity of the heart cells to sustain the constant calcium cycling needed to keep the heart going. The team tracked bluefin tuna in the wild using archival tags, which measured the depth of the fish, its internal body temperature, and the ambient water temperature. Using the wild data, the researchers then created experimental conditions in the lab with single tuna heart cells to see how they would respond. The research is helping scientists better understand how animals survive under drastic temperature changes, information that’s critical in a world being altered by climate change.