Scales in Fishes
The term Scale is derived from the Old French “escale” meaning a shell pod or husk.
Fish scales are part of the Fish’s integumentary system, and these are originated from the Mesoderm layer of the dermis.
A fish scale can be defined as the rigid plate that grows out of the skin of a fish. In most of the fishes, the skin is covered with these protective scales. These scales also provide effective camouflage through the use of reflection and coloration, as hydrodynamic advantages.
The scales vary in shape, size, structure, and extent that ranging from strong and rigid armor plates in fishes like shrimp fishes and boxfishes, in some, it is microscopic or absent such as Eels, and Anglerfishes.
Types of Scales in Fishes:
- Thelodont Scales
- Cosmoid Scales
- Placoid Scales
- Elasmoid Scales
- Ganoid Scales
- Leptoid scales
- Cycloid Scale
- Ctenoid Scales
- Modified Scales
These are the bony scales of Thelodonts, the most abundant form of fossils fishes.
Cosmoid scales are found only on ancient lobe-finned fishes including some earliest lungfishes (Dipnoi) and Crossopterygii. They were probably derived from a fusion of placoid scales. Cosmoid scales increase in size through the growth of the lamellar bone layer.
These are also called denticles. The placoid scales are structurally homologous to the vertebrate teeth “denticle” with a central pulp cavity supplied with blood vessels, that is surrounded by a conical layer of dentine, all of which sits on top of a rectangular basal plate that rests on the dermis.
- Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size.
- The outermost layer is composed of vitrodentine, a largely inorganic enamel-like substance
- In Shark, the skin is almost covered by small placoid scales, supported by spines.
- During forward movement in sharks, these scales create tiny vortices which reduce hydrodynamic drag and reduce turbulence, that makes swimming more efficient and quieter compared to bony fishes.
- Placoid scales also serve a role in anti-fouling by exhibiting the Lotus effect.
Example of Fishes with Placoid Scales:
- These scales are found in cartilaginous fishes as such Sharks, rays, and chimeras.
Elasmoid scales are thin, imbricated scales composed of a layer of dense, lamellar collagen bone named isopedine. A layer of tubercles usually composed of bones is found above it.
Examples of fishes with Elasmoid Scales:
- Eusthenopteron (Eusthenopteron is a genus of prehistoric sarcopterygians which has attained an iconic status from its close relationships to tetrapods).
- amiids, and teleosts, whose cycloid and ctenoid scales represent the least mineralized elasmoid scales.
- In Zebrafish, elasmoid scales are used in the lab to study the bone mineralization process and can be cultured (kept) outside of the organism.
Ganoid scales are derived from cosmoid scales. Ganoid scales are covered with a hard enamel layer like dentine and a layer of inorganic bone salt called ganoine instead of vitrodentine.
- Ganoine is a characteristic component of ganoid scales.
- In appearance, it is glassy, often multi-layered mineralized tissue, which covers the scales as well as the cranial bones and fin rays in some non-teleost ray-finned fishes.
Examples of Fishes with Ganoid Scales: Sturgeons, paddlefishes, gars, bowfin, and bichirs.
Leptoid Scales: (Bony – ridge)-
Leptoid scales are found in higher-order bony fish the teleost. The outer part of leptoid scales fan out with bony ridges and the inner part is crisscrossed with fibrous connective tissues.
- These scales are thinner and more translucent than other types of scales.
- Leptoid scales lack the hardened enamel-like or dentine layers.
- Leptoid scales overlap in a head-to-tail configuration, like roof tiles, making them more flexible than cosmoid and ganoid scales
Leptoid scales are further divided into two types:
Cycloid scales: It is a circular scale with a smooth texture and uniform with a smooth outer edge or margin. They are most common in fishes with soft-fin rays, such as Salmon and carp.
Ctenoid or toothed scales are like cycloid scales, but there is an exception that they have small teeth or spinules (small spines or thorns) called ctenii found on their outer or posterior edges.
- Because of the presence of these small teeth or spinules, ctenoid scales have a rough texture.
- These fins are usually found in fishes that possess spiny fin rays such as Perch like fishes.
The Ctenoid scales are further divided into three types:
- Crenate Scale: In this type of scale, the margin of the scale bears indentations and projections
- Spenoid Scales: The scale bears spines that are continuous with the scale itself.
- True Ctenoid Scales: The spines on the scales are distinct structures.
Example of Fishes with Ctenoid Scales: Most ray-finned fishes possess ctenoid scales.
The modified scales serve various functions. It is found in different types of fish. Like lateral lines possess a system of mechanoreceptors to detect water movement.
Examples and functions of modified scales:
- In bony fishes, the scales along the lateral line have central pores that allow water to contact the sensory cells.
- In dogfish sharks and chimeras- the dorsal fins spines. In stingrays, the stinging tail spines, and in ‘saw’ fish the saw teeth are examples of fused and modified placoid scales.
Some fishes without scales:
- Jawless fishes: Lampreys and hagfishes have smooth skin without scales and without dermal bone. For the protection of threat, the hagfishes exude copious quantities of slime or mucus.
- Most eels lack scales, though some species are covered with smooth tiny cycloid scales.
- Most catfish lack scales
- Mandarinfish (mandarin dragonet, is a small, brightly-colored member of the dragonet family, which is popular in the saltwater aquarium trade) lack scales, and they possess layers of smelly and bitter slime which blocks out disease and probably discourage predators, implying their bright coloration is aposematic.
You can also read:
- Migration in Fishes
- Scoliodon – Dogfish, Classification, and Characteristics
- Herdmania- Sea Squirts
Reference: Kotpal: Vertebrata
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