Athletes are always looking for that extra edge -- a way to beat the other guy by running a little faster or hitting a little harder. A fish that looks like a silver plate standing on its rim has found its own edge: it warms its body to well above the surrounding water temperature. That should allow it to swim faster and longer, improve its vision, and keep its organs in good shape as it cruises through cool, dark waters.
Opah -- also known as moonfish -- are found around the world, usually at depths of about 150 to 1200 feet. They can be as big around as an automobile tire, and they have silvery skin and bright red fins. They vigorously flap the fins on the sides of their bodies, generating heat.
Most fish quickly lose their body heat, so they stay at about the same temperature as the surrounding water. But a study in 2015 found that opah retain their body heat. On average, they were about nine degrees Fahrenheit warmer than the water, even though they seldom came to the surface to warm up.
The opah retains its heat in a couple of ways. First, layers of fat insulate the muscles that generate body heat, the heart, and other structures. And second, it has an intricate network of blood vessels around the gills. As water flows through the gills, it cools most fish. But in the opah, vessels carrying warm blood from the heart intertwine with vessels carrying cold, oxygen-rich blood from the gills. The warm vessels warm the blood in the cool vessels -- keeping this odd-looking fish toasty warm.