#lampridae

Opah or Moonfish (Lampris guttatus), family Lampridae, order Lampriformes, found in oceans around the world

This fish is capable of whole body endothermy (warm-bloodedness).

This is the only fish that is known to be capable of this. Some other fishes, like tunas and some sharks, are capable of "regional endothermy".

NOT TWO, BUT SIX SPECIES OF OPAH! 

Opah or moonfishes (genus Lampris)  currently  comprises  two  species,   Opah (Lampris  guttatus) and Southern Opah (Lampris immaculatus) presently known in both hemispheres in all oceans from tropical, temperate and sub-polar waters. But a new genetic and morphometric study describes three news species, and resurrects a opah species described in 1830, we have now 6 new opah species. 

As they are present in fisheries, fish buyers, initially noted variations in the appearance of opahs. Further  investigation  of  external  characteristics revealed that some specimens had a noticeably smaller eye diameter relative to body length and possessed variable spotting patterns and body pigmentation, proving that there were more than two species.

Though no directed fishery currently exists for opahs, their common  occurrence as bycatch and growing culinary popularity make these fishes a valued, and thus retained, addition to commercial fisheries. Also, opah (Lampris guttatus) have the ability of circulate warm-blood throughout the body, so it is the only known fish that has warm-blood. The endothermy in opah differs from other endothermic fishes in that they are able to circulate warm blood throughout their entire body and thus being the only fish group known that is able to elevate the temperature of their heart. This allows them to maintain enhanced physiological function at depth without returning to the surface to warm the heart.

Opahs often exhibit rapid vertical diving behavior, recent electronic tagging in the Pacific Ocean has documented Lampris spp.from the surface to depths in excess of 500 m.  

Image: Species of opah (in order)   Southern Opah Lampris immaculatus, common opah Lampris guttatus, Southern Spotted Opah Lampris australensis, East Atlantic Opah Lampris lauta, Bigeye Pacific Opah Lampris megalopsis and Smalleye Pacific Opah Lampris incognitus.