The bodies of most fish are scaly. The scales protect them from predators and rough surfaces, improve streamlining, and ward off diseases and parasites. But a few species have different forms of protection: tough skin, bony plates, or thick layers of slime.
Scales vary in size, shape, alignment, and structure. The bodies of most sharks, for example, are covered by rows of tiny, pointed scales that aim toward the tail. The scales of tarpon, on the other hand, are round and can be more than two inches across.
Scales are especially helpful for marine biologists. The scales grow throughout the fish’s life. Each year of growth is bounded by a dark ring, allowing scientists to determine a fish’s age. They can also analyze the scales to learn more about a fish’s growth rate, diet, and its environment from year to year.
The list of fish without scales is pretty short. It includes hagfish and lampreys—odd-looking critters that are covered in a thick layer of slime. Hagfish can shed that layer if they’re attacked, allowing them to simply slip away. The slime produced by clingfish contains a toxin that can kill other fish.
Moray eels have an especially thick skin. It’s covered with slime-producing cells, providing two layers of protection.
And a skinny species known as razorfish, which floats nose-down, is covered by bony plates that extend all the way to the tail, ending with a sharp spine.
So these and other “naked” fish have good protection even without scales.