Scales of fish

Scales are the prominent structure present on the outer surface of a fish body. It helps fishes in providing insulation and also plays an important function in protecting them (ie., fishes) from various kinds of external environmental stresses.

Types of fish scales with diagram

There are generally five different types of scales present on fishes which are discussed elaborately below along with their respective diagrams:

i) Placoid scale:

1. These type of scales are small, pointed and tri radiate in shape and size.

2. Denticles are found embedded in the dermal layer of their skin.

3. It consists of a diamond-shaped or rhomboidal-shaped basal plate, having an opening of the pulp cavity and flat trident spine.

4. Their basal plate remains embedded in the dermis which is held by sharpey’s and other connective tissue fibres and formed of a trabecular calcified tissue, the cement.

5. Their spine is composed of a hard-calcareous substance and their dentine is coated externally with a hard and dense enamel, such as vitrodentine.

6. The pulp cavity of their basal plate and spines contains dentine forming cells, the odontoblast, blood vessels, nerves and lymph channels.

7. Examples of fishes having placoid scales are: sharks, skates and rays.

ii) Ctenoid scale:

1. These type of scales are generally soft and flat dermal plates whose groove-like part is firmly attached and hind teeth bearing part remains free.

2. It is composed of a central thicker part called ‘nucleus’ and numerous concentric lines of growth for finding the age of fish.

3. On their posterior end numerous longitudinal grooves are present which are used for sucking nourishment from the fish skin.

4. Pulp cavity and dentine are absent in them.

5. These are the derivatives of ganoid scales in which ganoin layers, cosmine layers and bone cells are absent.

6. Examples of fishes having ctenoid scales are: perch and sunfish.

iii) Cycloid scale:

1. These type of scales are thin, flexible, translucent dermal plates.

2. It is roughly circular in shape and is composed of a central thicker part called ‘nucleus’ and numerous concentric lines of growth which are used to find the age of fish.

3. Their anterior border is more or less rounded and remains exposed.

4. Their posterior part has numerous longitudinal grooves for sucking the nourishment from the fish skin.

5. It is composed of a thin upper layer of bone and lower layer of fibrous connective tissue. 

6. Pulp cavity is absent in them.

7. These are the derivatives of the ganoid scales in which ganoin layers, cosmine layers and bone cells are absent.

8. Examples of fishes having cycloid scale are: salmon, carp and other soft fin rayed fish.

iv) Ganoid scale:

1. These are flat basal-looking scales.

2. They do not overlap instead they fit together like pieces of a jigsaw puzzle.

3. They have bony base layer similar to that of cosmoid scale.

4. Their outer layer is made up of inorganic bone salt called ganoin.

5. They are diamond-shaped and interconnected.

6. They act like armour to form a nearly impenetrable barrier to predators.

7. Examples of fishes having ganoid scale are: gars, bichirs and reed fish.

v) Cosmoid scale:

1. These scales have external spines composed of vitrodentine.

2. They have two basal plates.

3. Their inner layer is made of lamellar bone.

4. On the top of this layer lies a layer of spongy or vascular bone and then a layer of dentine-like material called cosmine.

5. These scales grow along with fish.

6. Example of fishes having cosmoid scale are: lobefins and lungfish.