This past weekend a man caught a very rare fish, called an opah (aka moonfish), off the coast of Ocean City, Maryland. What makes this fish so rare is that it is the only known warm-blooded fish. In fact, they are able to keep their whole body about 5 degrees Celsius above the water they swim in.
Other fish lose heat generated from muscle movements to the environment as the blood flows through the gills, which are exposed to the seawater. Opah have what are called retia mirabilia, or an intricate network of veins and arteries in their gills that allow the veins to transfer heat to the arteries as opposed to losing heat to the environment. This special adaptation allows opah to hunt and live in deeper waters. According to a quote from Nicholas Wegner from the National Oceanic and Atmospheric Association who studied these fish, “Just from looking at it, I really thought it was a slow, sluggish, deep-water fish that doesn’t do very much. But all indications are that this is a very fast fish and an active predator. We’ve put some tags on them to show that they migrate thousands of kilometres.”
Wegner, Snodgrass, Dewar, Hyde. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science. 2015. http://dx.doi.org/10.1126/science.aaa8902