Fish scale

A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as well as possible hydrodynamic advantages. The term scale derives from the Old French 'escale', meaning a shell pod or husk.The alligator gar has a tough armouring of rhomboidal-shaped ganoid scales The sturgeon has rows of ganoid scales enlarged into scute-like armour platesThe ganoid scales on a bowfin are reduced in size and resemble cycloid scalesEarrings made from the ganoid scales of an alligator garFossil of a primitive rayfin with ganoid scalesGanoid scales on a fossilised Lepidotes, circa. 130 myaPorcupine fish have scales modified into spinesPufferfish spines are not modified scales, but developed by an independent gene networkMandarinfish lack scales and protect themselves with a layer of smelly and bitter slime.Boxfish have plates of ossified skin fused together to form a rigid shellSeahorses have thin skin stretched over bony plates arranged in ringsSome filefish appear scaleless because their scales are so small.To casual examination tuna seem largely free of scales, but they are not. A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as well as possible hydrodynamic advantages. The term scale derives from the Old French 'escale', meaning a shell pod or husk. 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. 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. Cartilaginous fishes (sharks and rays) are covered with placoid scales. Some species are covered instead by scutes, and others have no outer covering on part or all of 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. Most fish are also covered in a protective layer of mucus (slime). The bony scales of thelodonts, the most abundant form of fossil fish, are well understood. The scales were formed and shed throughout the organisms' lifetimes, and quickly separated after their death. Bone, a tissue that is both resistant to mechanical damage and relatively prone to fossilization, often preserves internal detail, which allows the histology and growth of the scales to be studied in detail. The scales comprise a non-growing 'crown' composed of dentine, with a sometimes-ornamented enameloid upper surface and an aspidine base. Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish. Beyond that, there appear to be five types of bone-growth, which may represent five natural groupings within the thelodonts—or a spectrum ranging between the end members meta- (or ortho-) dentine and mesodentine tissues. Each of the five scale morphs appears to resemble the scales of more derived groupings of fish, suggesting that thelodont groups may have been stem groups to succeeding clades of fish. However, using scale morphology alone to distinguish species has some pitfalls. Within each organism, scale shape varies hugely according to body area, with intermediate forms appearing between different areas—and to make matters worse, scale morphology may not even be constant within one area. To confuse things further, scale morphologies are not unique to taxa, and may be indistinguishable on the same area of two different species. The morphology and histology of thelodonts provides the main tool for quantifying their diversity and distinguishing between species, although ultimately using such convergent traits is prone to errors. Nonetheless, a framework comprising three groups has been proposed based upon scale morphology and histology. Comparisons to modern shark species have shown that thelodont scales were functionally similar to those of modern cartilaginous fish, and likewise has allowed an extensive comparison between ecological niches. True cosmoid scales are not found on extant fish. They are found only on ancient lobe-finned fishes, including some of the earliest lungfishes. They were probably derived from a fusion of placoid scales. The inner part of the scales is made of dense lamellar bone called isopedine. On top of this lies a layer of spongy or vascular bone supplied with blood vessels, followed by a complex dentine-like layer called cosmine with a superficial outer coating of vitrodentine. The upper surface is keratin. Cosmoid scales increase in size through the growth of the lamellar bone layer.