Scientists identify world's first fully warm-blooded fish

Scientists identify world's first fully warm-blooded fish
The beautiful, predatory moonfish can warm its own body, just like mammals and birds.

Moonfish, or "opah" as they are often called, are large, oval-shaped deep sea predators. Their odd body shape might make them look like cumbersome swimmers, but they are actually extremely fast, able to hold their own against other speedsters in the ocean like tuna and swordfish. Unlike tuna and swordfish, though, which need to regulate their internal body temperature by swimming back and forth between cold and warm waters, moonfish have an advantage: They can stay down in the frigid deep waters.
 
Researchers first suspected something fishy (or very un-fishy, depending on your outlook) was going on when they studied the gills of the moonfish. They found that some blood vessels there — incidentally the blood vessels carrying warm blood from the body — were wrapped around vessels carrying cold oxygenated blood from the gills. This system, which is basically a counter-current heat exchange system, recycles heat generated from body movement. 
 
As a result of this remarkable system, moonfish can maintain their internal body temperature. This is quite different from the processes that allow some fish, such as sharks, to temporarily warm parts of their body as they move. The moonfish can maintain an elevated internal body temperature, even around its internal organs. 


Source & further reading:
 http://www.mnn.com/earth-matters/animals/stories/scientists-identify-worlds-first-fully-warm-blooded-fish#ixzz3aHqq4aBg 

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