Opah

The opah is the only known fully warm-blooded fish that circulates heated blood throughout its body.

In 2015, researchers with the NOAA Southwest Fisheries Science Center revealed the opah, or moonfish, as the first fully warm-blooded fish.

Although not as warm as mammals and birds, the opah circulates heated blood throughout its body, giving it a competitive advantage in the cold ocean depths from 150 to 1,300 feet below the surface.

The opah is equipped with specialized blood vessels that carry warm blood to its gills to rewarm the blood that cools as the fish breathes and absorbs oxygen from the water.

The opah is one of the most colorful of the commercial fish species, and is particularly popular in Hawaii.

2003, Margaret M. Smith, Phillip C. Heemstra (editors), Smiths' Sea Fishes (Revision of 1977, James Leonard Brierley Smith, The Sea Fishes of Southern Africa), page 398, A member of the lower epipelagic community, the opah feeds on squid and fishes (including some benthic species) and is usually found well offshore.

For a long time, opah has been thought to be good luck so it was only given away, never sold. Opah is a nonschooling open-ocean fish so it's not caught in great quantities.

Opah is an extraordinary-looking ocean fish, with an almost perfectly round shape; a comparatively thin, flat profile; a silvery skin, and bright crimson fins.