A fish is able to maintain a warm body temperature in deep, cold waters.

Credit: Ralph Pace

Some species such as tuna can keep parts of their bodies warm, but Nicholas Wegner of the National Marine Fisheries Service in La Jolla, California, and his colleagues report that the deep-swimming opah (Lampris guttatus; pictured) can make its entire body — including the heart — warmer than its environment by 3–6 °C. They measured the temperature and studied the anatomy of 22 opahs captured off the coast of California at depths of 50–300 metres. They found that the animal generates heat by flapping its pectoral fins and retains it using specialized blood-vessel structures in the gills.

This warmth probably boosts the power output of the fish's muscles, the authors say.

Science 348, 786–789 (2015)