Opah, Lampris guttatus. It's a endothermic fish (warm-blooded), with a rete mirabile in its gill tissue structure, first described in 2015 as exhibiting counter-current heat exchange in which the arteries, carrying warm blood, from the heart, warm the veins in the gills carrying cold blood.The gills are cooled by contact with cold water. The opah's pectoral muscles generate most of its body heat. The opah retains heat with insulating layers of fat, which insulates the heart from the gills, and t