Fish scales constitute the external covering of almost all fish species. There are four different types of fish scale, each with their own characteristics and variations. The structure and configuration of scales can be used to determine the species from which they came. The type of scale will affect the behavior of a fish--larger, heavier scales providing more protection but restricting movement, and smaller, lighter scales offering more freedom of movement but less protection.
Fish scales are produced in the mesoderm layer of the skin. This is located between the outer skin (the epidermis) and the underlying tissues. This distinguishes them from reptile scales which are formed in the epidermis. The genes that are responsible for the development of scales in fish are the same genes that result in hair and tooth development in mammals.
Placoid scales are formed of a rectangular base plate that is embedded within the skin of the fish and some of spine externally. The interior of the scale is a pulp that receives blood from the fish's vascular system, while the outside is made of an enamel-like substance called vitrodentine. The shape of the spines can vary greatly depending on species. However, almost all give the fish a rough texture. Sharks and rays are examples of fish with placoid scales.
It is generally accepted that cosmoid scales evolved from placoid scales fusing together. This is because cosmoid scales have two base plates and similar external spines composed of vitrodentine. The base plates are made from bone and new bone is added as the fish grows. Lungfishes and coelacanths have cosmoid scales.
Ganoid scales have a bony base layer similar to that of cosmoid scales. Indeed, ganoid scales are modified cosmoid scales. However, they differ in that their outer layer is made of an inorganic bone salt called ganoine and that they are diamond-shaped and interconnected. Between ganoid scales are peg-and-socket joints that articulate. Ganoid scales are found on sturgeons, bowfish, paddlefishes and gars.
Cycloid And Ctenoid Scales
Cycloid and ctenoid scales have different shapes but the same composition and positioning. Both are composed collagen and calcium carbonate, rather than bone, and both are overlapping. This means that they are more flexible than the other types of scales. While the edges of cycloid scales are smooth, those of ctenoid scales have tiny teeth-like protrusions called ctenii, giving them a rougher texture. The majority of bony fish have cycloid or ctenoid scales.
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