Scales of Fishes, Modification and Uses of Scales

Scales of Fishes | Modification and Uses of Scales

In fishes, dermal scales are mesenchymal in origin. They are small, thin, cornified, calcareous or bony plates which fit closely together or overlap. As regards the arrangement of scales on piscine 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 minimize the friction of water. But some times 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 like- Chimaeras, Polydon and Acipenser, the scales are present in some localized areas. Primitive fossil fishes possess exoskeleton in the form of plates and scales which consist of three distinct layers. The innermost layer of isopedine, the intermediate one is spongy layer and outer layer is of dentine.

Types of Scales

Five types of dermal scales have been identified in fishes : cosmoid, placoid, ganoid, cycloid and ctenoid.

  1. Cosmoid scales:

    These do not occur in living fishes. These were characteristic of certain ostracoderms, placoderms, and extinct sarcopterygians (lobe finned fishes and lung fishes). These consisted of 4 distinct layers : an outermost thin enamel-like ganoine, thick dentine-like cosmine, spongy bone and innermost compact bone.

  2. Placoid scales:

    These are characteristic of elasmobranch fishes only. Each placoid scale consists of a backwardly directed spine arising from a rounded or rhomboidal basal plate embedded in dermis. Spine is made of enamel-like and basal plate of dentine-like bony material. A pulp cavity inside spine opens through basal plate. Placoid scales are closely set together in skin giving it a sandpaper like quality.

  3. Ganoid scales:

    Ganoid or rhomboid scales are thick, usually rhomboid or diamond-shaped plates closely fitted side by side, like tiles, Providing a bony armour to the fish. In some cases they may overlap. Ganoid scales are characteristics of chondrosteans (Polypterus, Acipencer) and holosteans (Leipidosteus) that’s why these are often called as ganoid fishes. Polypterus has palaeoniscoid ganoid scales which is composed of 3 layers: outer enamel like ganoine, middle dentine-like and inner bony isopedine. Lepidosteus has lepidosteoid ganoine and inner isopedine.

  4. Cycloid scale:

    Cycloid scales are thin flexible translucent plates, rather circular in outline, ticket in the centre and marked with several concentric lines of growth which can be used for determining the age of the fish. They are composed of a thin upper layer of bone and a lower layer of fibrous connective tissue. They overlap each other, each scale is embedded in a pocket of dermis. Cycloid scales are found in lung fishes, surviving dipnoans, some holosteans (Amia) and lower teleosteans such as carps, cods, etc.

  5. Ctenoid scale:

    These are characteristic of modern higher teleosteans such as perch, sunfish, etc. In form, Structure and management they are similar to cycloid scale. They are more firmly attached and their exposed free hind parts which are not overlapped, Beer numerous small com like teeth or spines. Intermediate types between cycloid and ctenoid scales also occur. Certain fish is such as flounders, may beae are both types, ctenoid scales dorsally and cycloid scales ventrally.

Modifications of Scales

Some features are totally naked or scalelessSuch as Torpedo (electric ray) and catfishes. In eels, they are minute and deeply embedded in dermis. In some fishes they become localized (e.g. chimaeras). In globe fish (Tetradon) and porcupin3 fish (Diodon), they develop into large Protective spines making it difficult to be swallowed. In trunk fish (Ostracion) form a complete bony box. In sturgeon (Acipenser) stout bony plates or scutes form a bony armour. Shark teeth are modified large placoid scales. Sting (barb) of a sting ray is a modified placoid scale. The little teeth on the elongated rosetrum of sawfish (Pristis) are formed by placoid scales. In basking shark (Cetorhinus) myriads of placoid scales become gill rakers. In Sea horse (Hippocampus) and pipe fish (Syngnathus) the scales become fused To form a protective Bony ring around the body.

Uses of Scales

Scales form a protective covering of exoskeleton of the body. 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. Pattern of scale is rather constant, for every species. Thus arrangement, number, form and structure of scales play important role in identification and classification of fish species.

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