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> About five degrees Celsius warmer than the water around it; not warm like us:-"All fish have two kinds of blood vessels in their gills: vessels carrying blood in from the body to pick up oxygen, and other vessels carrying oxygenated blood back out again. In the opah, the incoming blood is warm after circulating through the fish&apos;s body. This is because the opah swims by quickly flapping its pectoral fins, rather than undulating its body like many other fish do, to propel itself through the water &#151; a process that generates high heat. But outgoing blood, which has just been in contact with water in the gills, is cold. Wegner noticed that in the opah&apos;s gills, the two sets of vessels are tightly bundled against each other, so that the incoming blood vessels can warm up the outgoing blood before it goes anywhere else. This set-up, known as &#147;counter-current heat exchange,&#148; allows warm blood to be delivered throughout the body.-"Some other types of fish, such as tuna, have similarly designed blood vessels in certain parts of their bodies, allowing for &#147;regional endothermy&#148; &#151; warm-bloodedness that&apos;s limited to certain organs or muscles, such as the eyes, liver or swimming muscles. But the opah is the only fish scientists know of that has this design in its gills, where most fish lose the majority of their body heat to the surrounding cold water. By warming up the blood in the gills before it goes anywhere else, the opah achieves not just regional endothermy, but whole-body endothermy, according to the paper&apos;s authors. Testing showed that the opah is able to maintain a core body temperature about 5 degrees Celsius warmer than the surrounding water."