The primary purpose of scales is to give the fish external protection. There are four main kinds of scales, and numerous variations of each kind. Placoid Cosmoid Ganoid Cycloid and Ctenoid Different fishes, different scalation. It is interesting to think about the lifestyle and habitat of a fish, then look at its scales. In the brain teaser page below, the scales of five shark species are shown, two are slow swimming bottom-dwelling sharks, one is a generalist predator, and two are fast swimming pelagic species.
Scale sizes vary greatly between species.
Some fishes, such as the freshwater eels have tiny embedded scales. Fishes such as the tunas have tiny scales often found in discrete areas of the body. Many fishes such as the snappers have medium sized scales whereas the scales of others such as the Tarpon, Megalops cyprinoides are large enough to be used in jewelery. The scales of the Indian Mahseer, Tor tor are known to reach over 10cm in length. As cycloid and ctenoid scales increase in size, growth rings called circuli become visible.