Fish scales are small, rigid plates that grow out of the skin of fish. They serve multiple functions such as protection against predators, parasites, and physical damage, and they also help in reducing water resistance when the fish swims.
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Types of Scales
Mainly scales are of 4 types: (i) Placoid scale (ii) Cycloid scale (iii) Ctenoid scale (iv) Ganoid scale
Some other scale types are – Imbricate scale, Leptoid scale, Cosmoid scale
1) Placoid scale:
(i) Small, tooth like scales
(ii) Consist of dentin-covered, pulp
Example:- Shark (Scoliodon sp.); Ray fish (Torpedo sp.) etc.
2) Cycloid scale:
(i) Circular scales with smooth edges.
(ii) Thin, flexible and often overlapping.
Example:- Most bony fish like Salmon (Salmo sp.), Carp (Cyprinus sp.) Rohu (Labeo rohita) etc.
3) Ctenoid Scale:
(i) Circular scales with small, comb-like projections.
(ii) Have a rough texture due to ctenii.
Example:- Some bony fish like Perch (Anabas sp.) and bass (Micropterus salmoides)
4) Ganoid scale:
(i) Rhomboid shaped scale with an enamel like outer layer
(ii) Composed of ganoin, hard shiny substances.
Example:- Bowfin fish (Amia calva), Sturgeons (Acipenser sp.) and gars
5) Imbricate scale:
(i) Overlapping diamond shaped scales.
(ii) Vary in hardness and thickness.
Example:- Many bony fish
6) Leptoid scale:
(i) Elongated, overlapping scales.
(ii) Composed of a protein called lepidin.
Example:- Eels (Angullia sp.)
7) Cosmoid scales:
(i) Thick-rhomboid shaped scales.
(ii) Consists of a hard outer layer of cosmine and a bony inner layer.
Example :- In ancient fish like coelacanths.
Use of Scale in Determination of Age
Fish scales are commonly used to determine the age of fish and the process is known as scale ageing Sclerochronology.
Fish scales exhibit concentric bands or rings. The number of rings on a scale provides the age of fish and the space between the rings is proportional to the growth of fish.
Each ring or band in a fish scale corresponds to a year of growth, and the process of forming these annual increments is called annuli formation.
Scales are usually taken from specific regions – near the dorsal fin or the lateral line.
Use of Scale in Classification of Fish
Scales play a crucial role in the classification of fish, helping the scientists and researchers in identifying different species.
Scale Type: Different species of fish possess specific types of scales – cycloid, clenoid, ganoid, or placoid. The presence of these scales help in classifying fish into broader categories.
Scale Shape and Arrangement: The shape and arrangement of scales provide information for classification at family, genus or species.
Scale size and coverage: Some fish have scales covering the entire body while mothers in specific regions aid in classification.
Scutes: Fish with specialized scales called scutes found near the lateral line on the dorsal surface contribute to the classification.
Scale development: The development of scales during different life stages is considered. Juvenile and adult scales may differ.