Fish scales are typically divided into four main groups, one of which is comprised of ctenoid and cycloid scales. Both of these types of scales are similar in composition, consisting of a rigid surface layer primarily made of crystallized calcium-based salts and a deeper, fibrous layer predominantly composed of collagen. They differ, however, in shape. Cycloid scales feature a smooth posterior edge, but ctenoid scales display ctenii, bony, comb-like structures that decorate the outer margin of the scale. Ctenii may exhibit a wide range of morphologies that vary by species.