The opah utilizes retia mirabilia to conserve heat, making it the newest addition to the list of regionally endothermic fish. Blood traveling through capillaries in the gills must carry cold blood due to their exposure to cold water, but retia mirabilia in the opah's gills are able to transfer heat from warm blood in arterioles coming from the heart that heats this colder blood in arterioles leaving the gills. The huge pectoral muscles of the opah, which generate most of the body heat, are thus able to control the temperature of the rest of the body.[8]