Scales in Fishes

The term Scale is derived from the Old French “escale” meaning a shell pod or husk.

Fish Scale

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 colouration, as hydrodynamic advantages.

The scales vary in shape, size, structure, and extent that ranging from strong and rigid armour plates in fishes like shrimp fishes and boxfishes, in some, it is microscopic or absent such as Eels, and Anglerfishes.

Various types of Scales in Fishes:
  1. Thelodont Scales
  2. Cosmoid Scales
  3. Placoid Scales
  4. Elasmoid Scales
  5. Ganoid Scales
  6. Leptoid scales
  7. Cycloid Scale
  8. Ctenoid Scales
  9. Modified Scales

 

Thelodont Scales: This is bony scales of Thelodonts, the most abundant form of fossils fishes.

Cosmoid Scales

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.

Cosmoid scales in Fishes- Queensland lungfish

                                                   Cosmoid scales in Fishes

Placoid Scales 

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.

 

Electron microscope view of placoid scale - Scales in fishes

                  Electron microscope view of the placoid scale

  • 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 shark, these scales create tiny vortices which reduce hydrodynamic drag and reduce turbulence, that makes swimming more efficient and quitter compare 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.
placoid scales in fishes

                                      Tiger Shark with placoid scale

Elasmoid Scales:

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.

Example of fishes with Elasmoid Scales:

  • Eusthenopteron (Eusthenopteron is a genus of prehistoric sarcopterygian 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

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.

Example 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 carps.

Ctenoid Scales:

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:

  1. Crenate Scale: In this type of scale, the margin of the scale bears indentations and projections
  2. Spenoid Scales: The scale bears spines that are continuous with the scale itself.
  3. True Ctenoid Scales: The spines on the scales are distinct structures.

ctenoid scales fishes

Example of Fishes with Ctenoid Scales: Most ray-finned fishes possess ctenoid scales.

Modified Scales

The modified scales serve various functions. It is found in different types of fish. Like lateral lines possesses a system of mechanoreceptors to detect water movement.

Examples and function 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 shark and chimaeras- 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:
  1. 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.
  2. Most eels lack scales, though some species are covered with smooth tiny cycloid scales.
  3. Most catfish lack scales
  4. 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 discourages predators, implying their bright coloration is aposematic.
The mandrainfish - the fishes without scales: mandarinfish

                                Fishes without scales; Ex. mandarin fish

 

You can also read:

Reference: Kotpal: Vertebrata

https://en.wikipedia.org/wiki/Fish_scale#Ctenoid_scales

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