The opah, also known as the moonfish, is the first known fish to be completely warm-blooded. This explanation brings more understanding to scientists who have long been confused by how the opah lives. Their existence in waters of hundreds of feet below the surface of the ocean is not typical of its cold environment. Their hunting habits have left those who study marine life often with more questions than answers.
Their physical features include a large heart, plenty of muscular tissue, and their entire body containing warm blood. The combination of the quick flapping of their pectoral fins and consistent arrangement of their vessels allows their blood to be warmed from the fin movement. In most species, the blood cools as it passes through the gills, but not so with moonfish. So the opah does not experience temperature drop in their body because the blood vessels in their gills expand throughout their entire body, bringing warm blood to heat other parts of its body.
As a result, its muscle performance is greatly enhanced, and it doesn’t have to waste valuable hunting time rushing to the surface to warm up.