Why Do Fish Have Scales?
Most fish are covered with a layer of flat scales, which overlap each other like the shingles on a roof. The hard scales form a protective covering for the softer body beneath. The scales in turn are covered with a thin layer of skin.
The skin gives off a slimy substance that coats the fish’s body and helps protect it against infections and parasites, if any of the fish’s scales are lost by accident, new ones grow to take their place. Fish scales reflect light, making the fish shimmer like water and, hopefully, slip by predators unnoticed.
As the fish grows, the scales also grow by adding rings of new material around their edges. Scientists tell the age of a fish by examining its 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. Cartilaginous fishes (sharks and rays) are covered with placoid scales. Most bony fishes are covered with the cycloid scales of salmon and carp, or the ctenoid scales of perch, or the ganoid scales of sturgeons and gars. Some species are covered instead by scutes, and others have no outer covering on the skin.
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.