In many vertebrates, the exoskeletal covering of body is made of two types of scales- epidermal and dermal. Epidermal scales are cornified derivatives of the Malpighian layer of epidermis. They are well developed in terrestrial vertebrates such as reptiles, birds and mammals. Dermal scales are mesenchymal in origin and especially developed in the fishes.
They are small, thin, cornified, calcareous or bony plates which fit closely together or overlap. As regards the arrangement of scales on fish body is concerned, they are most often imbricated and thus, overlap like shingles on the roof, with their free margins directed towards the tail, so as to minimise the friction of water.
But sometimes total reversal of the pattern of arrangement is seen in some fishes. Among barbot (Lota) and freshwater eel (Anguilla) the pattern is mosaic rather than overlapping one another, they are separated minutely or meet their neighbours only at their margins.
Scales vary in size and shape in different species. The body of all fishes except members of family Siluridae and a few bottom dwellers is covered by scales. Scales are usually found covering entire body surface, but in some fishes such as Chimaeras, Polyodon and Acipenser the scales are present in localised regions or areas of the body.
Primitive fossil fishes of Silurian and Devonian periods possess exoskeleton in the form of plates and scales which consist of three distinct layers. The innermost layer consisted of a compact bone, isopedine, the intermediate or middle layer of spongy vascular bone and the outer layer is of dentine.
According to the mode of the their origin, there are two types of scales:
(i) Those which are formed due to the secretory activity of both epidermis and dermis, as the placoid scales of elasmobranchs and
(ii) Non-placoid scales that are derived from the dermis only as the scales of teleosts.
Structurally, the scales are classified as cosmoid, ganoid or rhomboid, placoid, cycloid and ctenoid, the last two are also called the bony ridge scales:
1. Cosmoid Scales:
The cosmoid scales are found in the extinct Crossopterygii and Dipnoi. The external layer of scale is thin and enamel-like and is called the vitrodentine. The middle layer is made up of hard, non-cellular, dentine-like material called the cosmine and contains a number of branching tubules and chambers.
The inner layer is made up of vascularised bony substance, isopedine. These scales grew by the addition of new isopedine material from below, along the edges. Cosmoid scales are not found in the living fishes. The living Dipnoi possess thin cycloid scales and have lost the cosmine layer.
2. Ganoid Scales:
These scales are characteristic of the primitive actinopterygians called the ganoid fishes, and are of various forms and structure. These scales are heavy and have an outer layer of hard inorganic, enamel-like material called ganoine.
The middle layer is cosmine containing numerous branching tubules. The innermost layer is thickest and is made up of lamellar bone, isopedine. These scales grow by the addition of new layers to lower as well as upper surface. These scales are usually rhomboid in shape and articulate by peg and socket joints.
The ganoid scales are best developed in the chondrosteans (Polypterus, Acipenser) and holosteans (Lepidosteus) so that these are often called ganoid fishes. In Acipenser, they are in the form of large, isolated, bony scutes and are present in five longitudinal rows at places where there is greater wear and tear.
In Lepidosteus, the scales are hard, polished, rhombic plates, fitting edge to edge, thus, forming a complete armour. Scales of this fish have lost the middle cosmine layer, thus, reducing their thickness and rigidity. The scales of Amia are even thinner, do not possess the ganoine layer and resemble typical cyloid scales.
3. Placoid Scales:
These scales are characteristic of elasmobranch fishes (sharks) only. Each scale has a disc-like basal plate embedded in the dermis and a spine projecting out through the epidermis. In structure, a placoid scale resembles a tooth. The spine has an external covering of enamel-like, hard, transparent material called vitrodentine.
This is followed by a layer of dentine enclosing a pulp cavity from which several branching dentine tubules radiate in different directions. The centre of the basal plate is perforated by an aperture to provide entrance to the blood vessels and nerves from the dermis. The placoid scales are closely set in skin but do not overlap each other and giving a sand paper-like quality to the skin.
4. Cycloid Scales:
The cycloid scales are thin, flexible, transluscent plates, rather circular in outline, thicker in the centre and marked with several concentric lines of growth which can be used for determining the age of the fish. They are found in a large number of teleostean fishes having soft rayed fins, such as Labeo, Catla, Barbus, Cirrhina, etc. The central part of the scale is called the focus and is the first part to develop. In many species, oblique grooves or radii run from the focus towards the margin of the scale.
They form a protective covering over the skin and project diagonally in an imbricating pattern. The part of the scale which is exposed to view in situ condition (posterior area) generally shows less distinct ridges or circuli and chromatophores are also sometimes attached to it. The anterior area lies embedded in the skin.
5. Ctenoid Scales:
The ctenoid scales are also circular and can be distinguished from the cycloid scales by having a more or less serrated free edge. Moreover, several spines are present on the surface of the posterior area of the scale. These scales are characteristic of modem higher teleosteans such as perch, sunfish, etc.
They are more firmly attached and their exposed free hind parts which are not overlapped, bear numerous small comb-like teeth or spines. Intermediate types between cycloid and ctenoid scales are also found on different parts of the body. Certain fishes, such as flounders, may bear both types, ctenoid scales dorsally and cycloid scales ventrally.
Modifications and Variations of Scales:
Some fishes are totally scaleless or naked such as Torpedo (electric-ray) and catfishes. In eels, they are minute and deeply embedded in dermis. In some fishes (chimaeras) they become localised. Jaw teeth in the sharks are regarded as modified scales. The teeth forming the saw of Pristis and the sting-ray are modifications of the scales.
The scales at the base of tail in the sturgeon fish, Acanthurus, are modified to forming sharp cutting blades. In coffer fish, Ostracion, the scales form polygonal bony plates articulating with one another to form a rigid protective box. In the globe-fish, Tetrodon and the porcupine fish, Diodon the scales are elongated to form spines for protection.
The jointed fin-rays, leptotrichia of the bony fishes are also considered to be modified scales. In the basking shark, Cetorhinus, the scales become modified to form gill-rakers. The scales along the lateral line of the fish become perforated to provide an exit for the canal. In the sea-horse. Hippocampus, and the pipe-fish, Syngnathus, scales are fused to form protective bony rings around the body.
Uses of Scales:
Scales form a protective covering of exoskeleton on the body. The structure of scales is useful in broad classification of the group. Thus, among the fishes, placoid scales are characteristic of Chondrichthyes, ganoid scales are found in the primitive bony fishes, while the cycloid and ctenoid scales are found in the higher teleosts. Sometimes, the scales are useful in separating orders and families.
Thus, siluroid fishes are scaleless and can be distinguished from cyprinoids. Scales form an important source of information about fossil fishes and in classifying kitchen middens of extinct men. Scales are useful in studying the food habits of piscivorous animals. In taxonomic work, several species are identified by counting the scales along the lateral line and round the body.
Cycloid and ctenoid scales are of considerable help in calculating the age of and growth rate of fishes. Many species undergo seasonal growth which is apparent from the lines of growth on the scale. In some species like Salmo, spawning marks can be seen on the scales, so that it is possible to find how many times a fish has spawned.
Scales grow throughout life in size with the fish. Growth results in concentric lines which make age determination possible in salmon, trout, bass and several other species. For every species, its scale pattern is rather constant. Thus, arrangement, number, form and structure of scales play an important role in identification and classification of fish species.