Why Do Most Fish Have Scales?
The skin of most fishes is covered with scales. Scales vary enormously in size, shape, structure, and extent, ranging from strong and rigid armour plates in fishes such as shrimpfishes and boxfishes, to microscopic or absent in fishes such as eels and anglerfishes. The morphology of a scale can be used to identify the species of fish it came from.
Fish scales are part of the fish’s integumentary system, and 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 placoid scales of cartilaginous fishes are also called dermal denticles and are structurally homologous with vertebrate teeth. It has been suggested that the scales of bony fishes are similar in structure to teeth, but they probably originate from different tissue. Most fish are also covered in a protective layer of mucus (slime).
Fish have scales as a protective coating for the skin. In fact, not all fish have them. But we usually think of a fish as a cold-blooded, aquatic animal that swims by means of fins, breathes by means of gills, and is covered with scales. Scales may be of four different kinds-placoid, ganoid, cycloid and ctenoid.
Placoid scales are long, spiny and tooth-like, and are made of enamel and dentine. These are found on fishes which have a back bone made of gristle, such as sharks and rays. Ganoid scales are rather like placoid scales but are mainly bony and covered with a kind of enamel called ganoin. These thick scales are found especially in garfish.
Cycloid scales are thin, large, round or oval scales arranged in an overlapping pattern. They are found in carps and similar fishes. Ctenoid scales are similar to the cycloid ones, but have spines or comb-like teeth along their free edges. These are found in the higher bony fishes, such as perches and sunfishes.
Elasmoid scales are thin, imbricated scales composed of a layer of dense, lamellar bone called isopedine, above which is a layer of tubercles usually composed of bone, as in Eusthenopteron. The layer of dentine that was present in the first sarcopterygians is usually reduced, as in the extant coelacanth, or entirely absent, as in extant lungfish and in the Devonian Eusthenopteron.
Elasmoid scales have appeared several times over the course of fish evolution. They are present in some lobe-finned fishes: coelacanths, all extant and some extinct lungfishes, some tetrapodomorphs like Eusthenopteron, amiids, and teleosts, whose cycloid and ctenoid scales represent the least mineralized elasmoid scales.
Cosmoid scales are found in several ancient lobe-finned fishes, including some of the earliest lungfishes, and were probably derived from a fusion of placoid scales. They are composed of a layer of dense, lamellar bone called isopedine, above which is a layer of spongy bone supplied with blood vessels.
The bone layers are covered by a complex dentine layer called cosmine and a superficial outer coating of vitrodentine. Cosmoid scales increase in size through the growth of the lamellar bone layer.