Mesothermic animals occupy a middle ground between cold-blooded and warm-blooded species. Mesotherms are able to generate and retain some body heat while still relying partly on their environment. This rare adaptation, found in less than 0.1% of all fish, is seen in species such as basking sharks, great white sharks, and bluefin tuna.
According to a new study led by researchers from Trinity College Dublin in Ireland, mesothermic fish use nearly four times more energy than cold-blooded fish of similar size. The ability to keep parts of their bodies warmer than the surrounding water gives these species a performance edge, but it comes at a cost.
As the oceans warm, that cost is rising. Warmer water speeds up the metabolism of mesothermic fish, increasing their food demands just as shifting ecosystems may be making prey harder to find. At the same time, higher temperatures can push these species beyond their thermal limits, forcing them toward cooler waters and shrinking their range. Researchers call this convergence of pressures a “double jeopardy.”
The study, which was recently published in the journal Science, found that the challenge is especially severe for larger fish. Bigger bodies retain heat more easily, making it harder to cool down. The researchers estimate that a one-ton mesothermic shark may struggle to regulate its temperature in waters above 62 degrees Fahrenheit.
As climate change continues to warm the oceans, suitable habitats for these powerful predators could shrink, putting some of the ocean’s most iconic species at greater risk.
