Scales
Thin plates of bone and enamel-like tissue that overlap like roof tiles to protect a fish's skin.
Also: fish scales
Most fish are covered in scales that grow from the skin and overlap from head to tail, so that water flows smoothly over them. There are several kinds. Bony fish mostly have thin, flexible cycloid or ctenoid scales; sharks have tooth-like placoid scales; gars and bichirs have heavy, shiny ganoid scales. Some fish, such as many catfish, have no scales at all, and others carry bony plates instead. Scales grow as the fish grows, leaving rings that can be counted, like tree rings, to estimate a fish's age.