The opah is recognized as the first known fully warm-blooded fish, maintaining body heat throughout its body while swimming in cold water.
In 2015, researchers reported that the opah can keep its heart, brain and swimming muscles warmer than the surrounding seawater. Its unusual circulatory system helps achieve this: blood vessels carrying cold, oxygen-poor blood from the gills run alongside vessels carrying warmer blood from the body. Heat transfers between the vessels before the blood reaches the heart and muscles.
This adaptation allows the opah to remain active in deep, chilly waters where many fish become sluggish. The fish is often called a moonfish and has a rounded, laterally compressed body with bright orange-red fins. Some tuna, billfish and lamnid sharks can also warm particular muscles or organs, but the opah was the first fish documented as maintaining elevated temperatures across its whole body. That distinction makes it different from merely partially warm-bodied fish.