The opah, otherwise known as moonfish, sunfish, kingfish, redfin ocean pan and Jerusalem haddock depending on who you ask, has been around for awhile and has recently become increasingly popular in seafood markets. However, it has never occurred to fishermen that there might be something strange about the opah, but it has been confirmed by researchers that it is, in fact, the first and only warm-blooded fish.
The study was conducted by researchers from the National Oceanic and Atmospheric Administration after they discovered that the opah demonstrated several key traits that went against other cold-blooded deep ocean fish. According to study author Nicholas Wegner, the key to the opah's warm blood lies in the uniquely formed blood vessels around the gills of the species that allow blood to circulate through the fish's entire body. Unlike most deep sea fish, the opah is not sluggish from the cold, but rather its warm blood gives it a distinct advantage through speed. Even in its home in the frigid North Pacific, it can keep its body temperature around 5 degrees Celsius warmer than the water it swims in. With large hearts, large eyes and lots of muscles they can thrive in any part of the open ocean. Due to their warm blood and abundance of muscles, the meat of an opah looks more like a slab of beef than a filet of fish when it is raw.
As the opah lives in the depths of the ocean, not much is known about its biology or their ecology, but scientists believe they live, breed and die in the open ocean, living on a diet of squid and krill while occasionally schooling with tuna. As these fish are so elusive, they are more likely to be scooped up in tuna nets rather than caught by recreational anglers.