Question

Tooth-shaped scales are
A. Cycloid
B. Ctenoid
C. Ganoid
D. Placoid

Answer 1

Hint: Scales give fish the protection externally. There are four types of scales. All of them differ from each other significantly. The scale sizes are not the same for all species. They vary depending on the type of fish species. However, many fish species are devoid of scales.

Complete answer:
> Option A: Cycloid- Their shape is circular. They have a smooth edge and are mostly found in the lower order teleost fishes like salmon and soft fin ray. The scales keep growing along with the fish. The growth of scales is also dependent on weather. For instance, in cool weather, these scales grow at a slower rate.

> Option B: Ctenoid- They are composed of a bony layer. They resemble the structure of a comb. Higher order teleost fish like sunfish have these types of scales. These scales grow with the fish by adding concentric layers to it.

> Option C: Ganoid – Their shape is rhomboid and they have articulating peg and socket joints between them. They have a bony basal layer, dentine layer and outer layer which is ganoine.

> Option D: Placoid – They are very tiny and sharp scales, resembling the structure of a tooth. They grow out of the dermis layer. These scales are present in shark, ray and other cartilaginous fishes.

Hence, the correct answer is option (D).

Note: Fish scales have many important functions. They protect the skin of fishes from attack by predators and parasites. The scales overlap with each other which thus provides a very flexible coating and fish movement while swimming becomes very efficient.