Normally, an upside-down fish in your tank is bad news. As in, it’s time for a new goldfish.
That’s because most fish have an internal air sac called a swim bladder that allows them to control their buoyancy and orientation. They fill the bladder with air when they want to rise, and deflate it when they want to sink. Fish without swim bladders, like sharks, have to swim constantly to keep from dropping to the bottom.
But what of Central Africa’s seven species of upside-down catfish? Their bodies don’t seem to be different from catfish that swim right-side up, yet they swim flipped. And that orientation provides some advantages, including better access to oxygen and the potential for reduced turbulence when swimming.