Scales in fishes:
- Scales are a visible outgrowth of the fish’s epidermis
- Word Scale is derived from the old French term “escale,” which means husk.
- Scales may come in a variety of shapes, sizes and arrangements.
Types of Scales:
Fish scales are classified into following categories, namely
- Placoid
- Ganoid
- Cycloid
- Ctenoid
| Type of Scale | Characteristics | Appearance | Examples of Fish Species |
| Placoid Scales | Small, tooth-like structures | Rough texture | · Sharks · Rays |
| Ganoid Scales | Thick, rhombus-shaped | Glossy, enamel-like | · Gars · Bichirs |
| Cycloid Scales | Smooth, round-edged | Flexible | · Salmon · Carp · Trout |
| Ctenoid Scales | Comb-like edges | Flexible | · Perch · Bass · Sunfish |
| Cosmoid Scales | Bony core with cosmine covering | Durable | · Coelacanths |
Let us discuss each type in detail:
Placoid Scales:
Placoid scales, also known as dermal denticles, found on the skin of cartilaginous fish such as sharks, rays and chimaeras.
| Structure | Characteristics | Function |
| · Base: Grooved, fits into skin pockets, connected to nerves. · Body: Tapered, tooth-like structure composed of dentine. · Crown: Enamel-like substance covering the exposed tip. · Nerve Connection: Nerves extend from the base to monitor vibrations.
| · Small, tooth-like structures on fish skin. · Composed of dentine and enamel-like material. · Embedded in the skin with the pointed tip exposed. · Scales decrease in size as they move towards the tail. · Typically have a smooth, streamlined shape.
| · These scales help in reducing water resistance, enabling efficient swimming. · Allows fish to glide smoothly through the water.
· Provide protection against predators and minor injuries. · Enhance sensory perception through connections to sensory cells. · Contribute to the distinctive texture of sharkskin leather.
|
Ganoid Scales:
Ganoid scales are found in primitive fish species like gars and bichirs.
| Structure
| Characteristics | Function |
| · Ganoid scales have a rhomboid or diamond like shape. · They are composed of a bony, enamel-like outer layer called ganoin, which is covered by a layer of skin.
| · Scales are relatively heavy compared to other scale types. · Scales are non-overlapping and form a protective armor-like covering.
· Ganoin layer is hard and shiny, often with a metallic appearance.
· The outer surface of the scales may be covered in a layer of mucus.
| · Provide excellent protection against predators due to their hard and rigid nature. They create a tough barrier around the fish’s body. · The relatively heavy nature of ganoid scales helps the fish to control its buoyancy in water. · The scales can help in maintaining body temperature by reducing heat exchange with the environment. · Some ganoid scales have patterns that aid in camouflage, making the fish blend into its surroundings.
|
Cycloid scales
Cycloid scales are one of the two main types of scales found in modern bony fish, known as teleosts, including many common freshwater and marine fishes.
| Structure
| Characteristics | Function
|
| · Cycloid scales have oval or a circular shape · They are composed of a thin, flexible layer of bone covered by a layer of skin.
| · Cycloid scales have smooth outer edges, which can reduce drag and allow for streamlined movement. · Cycloid scales often exhibit concentric rings, can be counted to estimate the fish’s age.
| · Cycloid scales offer protection against minor injuries and parasites. · The overlapping arrangement and flexible nature of these scales enable the fish to move freely and efficiently. · The smooth edges and coloration of cycloid scales can aid in camouflage by reducing light reflection and blending with surroundings. |
Ctenoid Scales:
Ctenoid scales are a type of fish scale found in modern bony fish, or teleosts.
| Structure
| Characteristics
| Function |
| · Ctenoid scales have a comb-like or toothed appearance along their posterior edge. | · Ctenoid scales have small comb-like projections along their rear edge. · These scales overlap like shingles on a roof, providing comprehensive coverage. · Ctenoid scales are found in various teleost fish species, in both freshwater and marine environments.
| · Ctenoid scales contribute to maintaining buoyancy without adding excessive weight to the fish. · The overlapping arrangement and thin structure allow for flexible movement during swimming. · The smooth side reduces drag, enhancing swimming efficiency and allowing swift movement. |