Fishes possess dermal scales on the body for protection. Each scale is made of dentine that is secreted by dermal papilla which is a group of specialized cells capable of drawing nourishment from neighbouring tissues to produce an organ. The exposed portion of scale is covered with a layer of hard enamel to minimise wear and tear.There is a range of variety of scales found in fishes. Ancient fishes generally had thick bony scales while the modern fishes have evolved thin and flexible scales for more agility.
KINDS OF SCALES
The following types of scales are found in living and extinct fishes:
They were found in ancient crossopterygian fishes of Devonian period and covered the body like thick bony blocks, more or less like crocodiles have today. A cosmoid scale is made of a thick layer of cosmine that is dentine type of material covered on the surface by a hard proteinous layer called vitrodentine. The surface has a pattern of pits (lacunae) and canals (canaliculae). Under the cosmine, there is a thick layer of spongy bone that carries blood vessels for the growth and nourishment of the scale. The last layer close to the body is called isopedine which is softer and spongy lamellar bone.
They are thick, rhomboidal scales having the surface coated with a hard enamel-like material called ganoin and lacunae (pits) and canaliculae (fine canals) forming the surface sculpture. Bulk of the structure is made of the dentine-type bone and a lower layer that is called isopedine or lamellar bone. Such types of scales are called Palaeoniscoid type that occurs in Chondrostei such as Polypterus. In modern Holostei, such as Amia and Lepidosteus dentine-like layer is lost and the scales have only isopedine coated on the surface with ganoin. This type of scales is called Lepidosteioid type.
CYCLOID AND CTENOID SCALES
These scales are found in modern teleosts and are thin, strong and extremely flexible. They are large, oval in shape and made of only isopedine with an underlying layer of collagenous fibres that provide them with strength and flexibility. Ctenoid scales teeth at the base that helps them to firmly fix in the skin tissue.
They are characteristics of cartilaginous fishes (Chondrichthys) and are hard and microscopic in size. Their body is made of dentine and the exposed surface is covered with a layer of hard enamel. There is a pulp cavity in which dermal papilla sits during the development of the scale. In structure placoid scales resemble a tooth and for the same reason, teeth of sharks are modified placoid scales that are anchored in dermis and are replaced throughout life.
Evolutionary modifications in scales of fishes
Ostracoderms and placoderms, which were ancestors of all modern fishes, had their bodies covered with dermal bony armour for protection from predators. In Devonian period some of these fishes gave rise to predatory sharks that lost dermal armour for smaller placoid scales in order to achieve flexibility of body to swim faster and chase prey.
They had powerful jaws that could crush the protective bony covering of body of other fishes. The dermal armour now lost its value in defending themselves against these new powerful predators. These fishes, therefore, had no option but to shed their heavy bony covering and evolve smaller and thinner scales to provide the body with flexibility for swimming faster and manoeuvre quickly to escape from predators. Hence the bony armour gave way to cosmoid scales which were still like thick bony plates covering the body.
In ganoid fishes, the spongy bone of cosmoid scales was lost and even dentine layer was lost in Holostei to make the scales thinner and more flexible. Cycloid and ctenoid scales of modern teleosts are very thin and flexible and consist of only isopedine to which a layer of collagenous fibres is attached ventrally for strength and flexibility. Owing to these changes in scales, the modern fishes are no longer sluggish creatures like their ancestors but have highly flexible body to swim in water with speed and agility.