Not all fish have scales. While scales are a characteristic of most fish, some fish, such as clingfish, lampreys and catfish, do not have scales.
There are four main types of scales and many different varieties. Sharks and rays have placoid scales, which increase in number to fill in spaces as the fish grows. Boney fish have cycloid and ctenoid scales, which are present on the fish when it is hatched and grow larger as the fish grows. These scales can be used to determine the age of the fish. Cosmoid scales are made of two layers of bone, and new bone is added to each scale as the fish grows. Ganoid scales are a modified version of cosmoid scales and are found on fish such as the sturgeon.