The scales of a fish are like small transparent pieces of bone which protect the fish from minor cuts and scratches and to reduce drag as the fish swims. Scales vary in size from the very tiny to large bony scutes and there are several types of scale found on different families of fishes.
|Cycloid scales||Ganoid scales||Bony scutes|
Ctenoid scales and Cycloid scales are essentially cycloid scales with teeth at their posterior edges. The purpose of the teeth is to reduce drag when the fish is swimming. The scales have two main areas, the surface which is bony which is a; framework filled with calcium salts and a deeper fibrous layer composed mainly of cologen. The scales overlap each other like roof tiles. As fish with cycloid or ctenoid scales grow their scales grow with them.
Ganoid scales are a non-overlapping or partially-overlapping scale, often rhomboidal in shape, with thick outer ganoine layer (enamel-like substance).
Placoid scales are tough scales that cover the skin of elasmobranchs (sharks and rays).
Bony scutes are less common, type of scale is the scute, which is a shield-like bony plate, or a modified, thickened scale which can be ridged or spined.
Ctenoid and cycloid scales are found on the majority of living bony fish today. Ganoid scales are found only on fish which are generally regarded as being more primitive sturgeons, paddlefishes, gars, bowfin, and bichirs. Placoid scales are only found on sharks and rays.
The scales grow in the dermis, the inner layer of the skin, and are covered by a fine epidermis or outer skin layer. Each scale fits into its own little pocket of epidermis. The outer skin (epidermis) contains mucus glands which secrete the mucus which gives fish their slippery feeling. The mucus has many properties including being anti fungal, anti bacterial and because of this it is able to protect the fish from infection. Due to its slippery nature it also makes it more difficult for parasites to get a grip on the fish. As well as protecting the fish's health the mucus cuts down on drag and makes swimming more efficient. Some primitive fish like hagfish are able to produce vast quantities of mucus which is used to deter predators.
The scales grow with the fish, they grow faster in summer when there is a greater abundance of food and warmer water and slower in winter when the fish are more torpid. This changing growth rate leaves visible rings on the scale and so when viewed under a magnifying lens the age of the fish can be estimated.
If a scale is lost due to a trauma of some kind it will re-grow as long as the base of the scale deep in the dermis remains intact. Some specialised scales if lost may be replaced with ordinary scales.
A recent discovery found that the shine or iridescence of fish scales is caused by so-called photonic crystals made of guanine in the skin beneath them, this had long been accepted but no one knew quite how this worked.
The epidermis layer covering the scales is extremely thin and could easily be damaged by poor handling, in a healthy fish the epidermis will quickly regenerate and the fish won't be any worse for it, however if the fish is under some stress i.e. conditions in the aquarium are less than perfect with high nitrates for example or if the fish is in poor condition due to being recently imported or purchased then it could easily allow infection to gain a foothold due to the loss of the protective mucus such an infection will prevent healing of the epidermis and act as a reservoir for the infection to multiply freely. It is therefore very important to take great care when handling fish in order to avoid damage and to treat any sign of infection as soon as it is noticed.
Scutes: Scutes are similar to scales and serve the same function. Unlike the scales of lizards and snakes, which are formed from the epidermis, scutes are formed in the lower vascular layer of the skin and the epidermal element is only the top surface. Forming in the living dermis, the scutes produce a horny outer layer that is superficially similar to that of scales.
Dermis: a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues
Epidermis: is composed of the outermost layers of cells in the skin.
Guanine: nucleotide binding proteins are glycoproteins anchored on the cytoplasmic cell membrane. They are mediators for many cellular processes