The unique thing about the moonfish isn’t the fact that it's as big as a car tire or comically round. It’s its blood. The opah, or moonfish, is the first species of fish identified by scientists that’s warm-blooded.
While tuna and some sharks can temporarily retain a bit of heat in the muscles they use for swimming [PDF], their bodies aren't fully warm-blooded. By contrast, the camera-shy and relatively little-studied opah keeps its whole body, especially its brain, at temperatures higher than its surrounding environment, as researchers from the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center in California discovered. This, they write in the journal Science, makes the opah “distinctively specialized to exploit cold, deeper waters” up to 1300 feet below the surface.
Blood vessels carrying warm blood from the fish’s heart are located directly next to those that carry cold blood in the gills, forming a counter-current heat exchange that works kind of like a car radiator. The warm blood from the fish’s core helps warm up the cool blood that has been closer to the cold water in the gills (where it absorbs oxygen).
This makes the big fish a fearsome predator. Thanks to the warm blood flowing throughout its body, it likely has better eye and brain function than its cold-blooded counterparts. While most other fish in cold environments move relatively sluggishly, the moonfish is active and agile, with more muscle power and stamina to chase down its deep-water prey.
[h/t: Science Daily]
All images courtesy NOAA Fisheries/Southwest Fisheries Science Center