Everyone knows that some fish have scales. But why? What purpose do the scales serve for the fish? Most scientists believe the primary purpose of scales is to give the fish external protection.
Scales come in several types and many sizes. The most common type of scales are named elasmoid, and they occur on bony fishes, i.e. those with internal skeletons of bone. These scales are like thin slivers of calcified material with a fibrous base of collagen that is anchored in the fish’s dermis. There are two types of elasmoid scales, cycloid and ctenoid. These names refer to the shape of the scales. In cycloid scales the outer edge is rounded, and in ctenoid scales it has a row of fine teeth. A fish can have all one type of scale or it can have both. The scales may not cover all of the fish's body.
Most elasmoid scales are overlapping, with the back edge of one scale covering the front edge of the next, and so forth. Elasmoid scales grow as the fish grows. As the scales grow in size they develop growth rings something like a tree has, and scientists can evaluate the age of the fish by studying the growth rings.
Scale sizes cover a wide range. The scales of eels are microscopic. At the other end of the spectrum are tarpon, which can have scales two inches wide. Smaller scales can provide greater flexibility. Think about how sinuous an eel looks as it moves. Scale size can also relate to the environment where a fish lives. Fish swimming in fast moving waters tend to have smaller scales than fish that swim in slow moving waters. Puffers and porcupine fish have spines that are really just modified scales.
Some fish have no scales at all. The ocean sunfish, or mola mola, is covered with a layer of mucous for protection rather than scales.
The other main category of fishes is the cartilaginous fishes, which includes sharks and rays. These fish have a skeleton of cartilage instead of bone. They have scales named placoid. Placoid scales do not grow as the fish grows. Instead, spaces open up in the fish’s dermis between the scales, and a new scale grows out to fill the gap. Placoid scales are also anchored in the dermis, but the shape of the outer portion varies greatly. Some of these scales are shaped more like spines and give the exterior of the shark a rough feel.
Placoid scales are bony, with a layer of dentine and an outer layer of enamel. The soft inner core is vascular, meaning that it is supplied with blood. Placoid scales can also be called denticles and have some similarity to teeth.
Just like mammals have evolved a variety of fur, hooves, horns, and fangs for protection, fish have evolved a variety of scales for protection. In fact, the same genes that are involved in tooth and hair development in mammals are involved in scale development in fish.