Question

Placoid scales are found in
a. Cartilaginous fishes/sharks/chondrichthyes
b. Bony fishes/osteichthyes
c. Lung fishes
d. All the above

Answer 1

Hint: Placoid scales are tiny, tough scales that cover the skin of cartilaginous fishes or elasmobranchs which include fishes like sharks, rays, etc. The placoid scales are somewhat similar to the scales of the bony fishes; they are like teeth covered with enamel.

Complete answer:
Placoid scales are also known as dermal denticles as they are produced from the dermis layer. Placoid scales are tightly packed together and are supported by the spines and are laid flat by growing with their tips facing backward. These tough scales help to protect the fish from the predators and can also be used to kill or injure the prey as they are hard and sharp. These scales are of v-shape which reduces drag and increases turbulence as the fish moves through the water so that they can swim more quickly. The swim suits are designed to mimic the composition of the placoid scales.

Fish skin has a flat rectangular base plate of placoid scale embedded in it which provides support to the fish body. There are different species of fish that support their scales with unique spines based on the shape and the role of the fish. Ray fishes are flat whereas sharks are angular therefore; placoid scales are different for both fish to swim quickly.

Hence, the correct answer is option (A).

Note: Placoid scales are structurally homologous with the vertebrate teeth as they have central pulp cavities which have blood vessels, surrounded by conical layers of dentine. Always remember placoid scales of chondrichthyes and ctenoid scales of osteichthyes.