Fish scales are produced from the mesoderm layer of the dermis, which distinguishes them from reptile scales. The same genes involved in tooth and hair development in mammals are also involved in scale development.
The outermost layer is composed of vitrodentine, a largely inorganic enamel-like substance. Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size.
were probably derived from a fusion of placoid scales.
Cosmoid scales increase in size through the growth of the lamellar bone layer.
Most are diamond-shaped and connected by peg-and-socket joints.
Cycloid and ctenoid scales
cycloid scales have smooth margins, while ctenoid scales have tiny teeth called ctenii on the posterior edge that give them a rough texture.
In flatfishes, some species have ctenoid scales on the eyed side and cycloid scales on the blind side, while other species have ctenoid scales in males and cycloid scales in females.
Ctenoid scales can be further subdivided into three types:
▪ In crenate scales, the margin of the scale bears indentations and projections.
▪ In spinoid scales, the scale bears spines that are continuous with the scale itself.
In "true" ctenoid scales, the spines on the scale are distinct structures.
Cycloid and ctenoid scales are overlapping, making them more flexible than cosmoid and ganoid scales. They grow in size through additions to the margin, creating bands of uneven seasonal growth called annuli (singluar annulus). These bands can be used to age the fish.