Types of Fish Scales

What are Fish Scales?

Most fishes have scales on their external body that are actually part of their exoskeleton covering. They protect the fish skin from the external environment and prevent the entry of pathogens. Fish scales are basically the secretion of skin that evolved independently in bony and cartilaginous fishes. We can say that the scales are complex derivatives of integuments.

Scales can be cornified, big or small; thick or thin; bony or calcareous, or a combination of both. The same gene responsible for hair and tooth development in mammals is also responsible for the scales in fishes.

The arrangement of the scales on the fish’s body is imbricated or overlapping, just like shingles or roof tiles. Their free margins are arranged towards the tail region so as to minimize the effect of friction while swimming. 

Size of Scales

The size of scales may vary in different kinds of fishes:

For Example:

• The freshwater eels possess microscopically visible minute scales.
• Mackerel have small or average-sized scales.
• Megalops or tarpons have large-sized scales that are useful in making junk jewellery.

Content: Types of Fish Scales

1. Functions of Fish Scales
2. Types of Scales in Fishes
   • Cosmoid Scales
   • Ganoid Scales
   • Placoid Scales
   • Cycloid Scales
   • Ctenoid Scales

Functions of Fish Scales

• Scales protect the fish’s body from external injuries and harsh environments. Also, they defend them against scraper and parasites.
• Since they are part of the exoskeleton covering, they are useful in the classification of fishes based on their external appearance.
• The scales have an imbricate arrangement; this makes them flexible to slide over one another while bending or swimming.
• In most cases, they also determine the age of the fish as well as its rate of growth.
• They give us in getting a clear view of the extinct fishes.
• Commercially, they are useful in the cosmetics and medicinal industries.

Types of Fish Scales

The scales of the fishes are dermal scales. We can broadly classify them into five categories:

1. Cosmoid Scales
2. Placoid Scales
3. Ganoid Scales
4. Cycloid Scales
5. Ctenoid Scales

1. Cosmoid Scales

The cosmoid scales are rare in today’s era as it was a characteristic feature of fishes that are already extinct. For instance, they were present in Oostracoderm, placoderms, crossopterygian and Dipnoid. Also, they were present on the bodies of lungfishes. Nowadays, they are present in rare species of coelacanths that are still alive.

Structure

They are nearly semi-circular in shape and arranged very close to each other. Their external coating is vitrodentine, which is thin but enamel-like. The next layer is cosmine. This middle coat is formed of a hard, non-cellular dentine-like substance. It consists of several branching tubules and chambers.

The innermost layer is of isopedine, which is a vascularized bony material. This is a very important layer as the cosmoid scales grow with the new addition of this layer along the edges.

2. Ganoid Scales

The ganoid scales are hard and thick and are a characteristic feature of primitive ganoid fishes. They are present in Chondrosteans and Actinopterygians. These scales keep growing in size along with the fish’s age. One fascinating thing about these scales is the varying layers in their composition. For instance, these scales in the dinosaur eels comprise three layers. Whereas in slender gar fishes, there are only two layers.

Structure and Arrangement

The ganoid scales are little diamond or rhomboid shaped. Their arrangement looks exactly like the tiles aligned on the floor, appearing like a bony armour on the fish skin. They barely overlap each other instead are articulated with peg and socket joints.

They are heavy and possess a hard-outer layer made up of an enamel-like substance called ganoine. The middle layer is cosmine having several branching tubules. The inner coat, isopedine, is the thickest one, made from lamellar bone.

The ganoid scales grow with the addition of fresh layers on the upper and lower surface.

Example

They are most nicely developed in Holosteans and Chondrosteans, which is why they are also referred to as ganoid fishes.

In Acipenser, which is a Chondrostean, they are present in the form of bony scute arranged in five longitudinal rows. They are large and isolated in places with greater wear and tear.

In Lepidosteus, which are Holosteans, the ganoid scales are rhombic plates fitting edge to edge, giving a complete armour look.

3. Placoid Scales

The placoid scales are considered to be the most primitive type of scale. They are prominent features of the skin of the shark (elasmobranch) and electric ray. They provide a unique texture to the fish’s body by giving it a sand-paper-like look. However, when you place these scales under a microscope, it is amazing to see the intricate little hook-like structure.

These scales remain of the same size and do not grow with the ageing of fish.

Structure

Under the microscope, they resemble the tooth structure. Each scale has a spine made of enamel; it remains pointed backwards. This spine comes out of the disk-like rhomboidal basal plate. For the entry of blood vessels and nerves from the dermis, the centre of the basal plate has an aperture. These scales never overlap each other.

The placoid scales have three layers.

• The innermost is a pulpy vascular core.
• The second layer is dentine.
• The outermost layer is Vitrodentine which is hard and transparent.

4. Cycloid Scales

As the name suggests, the cycloid scales are circular or ring-like in shape. These scales are useful in determining the age of the fish because the number of rings keeps increasing with age. This implies that the more the number of concentric rings, the older the fish will be.

Cycloid scales are thin and remain covered with a coating of mucus or epidermis. Due to this, the fish has a slimy appearance. The structure of cycloid scales is nearly circular ring-like. They are translucent, thin, and flexible but are thicker in the centre. Found in several teleosteans that have soft-rayed fins such as Minnows, Catla, Barbus, Cirhina, Labeo. The centremost portion of the scale is the focus which is the first part to develop.

Structure

They are entirely made of isopedine with collagenous fibres in the underlying layer. This layer provides the necessary strength and flexibility to the scales.

5. Ctenoid Scale

The Ctenoid scales are the most category of scales. The word ctenoid originated from ‘ctenidium’, which refers to anything that has a comb-like shape or design. Thus, these scales bear specific comb-like projections at their posterior end. Other than this structure, their arrangement is similar to that of cycloid scales. But, they remain more firmly attached to the dermal skin. They don’t overlap each other; only the comb-teeth are touching one another. The comb teeth projections make it easier for the fish to swim faster. These scales grow bigger in size as the fish grows, developing rings.

They can easily be distinguished by cycloid scales by more or less serrated free-edges. In addition to that, numerous spines are located on the surface of the posterior area of scales. Certain fishes have both cycloid and ctenoid scales on the same body. Sometimes either male may have ctenoid, and a female may have cycloid or vice versa.

We can divide the ctenoid scales into three sub-categories:

• Crenate scales: In these scales, the margin has indentations and projections.
• Spinoid Scales: They have a spine that continues with the scale.
• True ctenoid scales: The spine here are visibly distinct structure from the scale.

Note: The scales of fishes give them a distinct characteristic look and provide texture to their body. For edible fishes, these scales are removed first before consumption.