Question

Why do fishes have streamlined bodies?

Answer 1

This is a question that is actually linked to the symmetry of the organisms. The word ‘streamline’ means that the body is built up in a straight-line fashion. As we all are well-known to the bodies of fishes, they have a smooth body. The fishes are organisms that have bilateral symmetry. This bilateral symmetry has a major role in
contributing to the streamlined fashion of the fish’s body structure.

Complete answer:
The fishes live in aquatic environments. The fact to be noted here is that water is more viscous and denser than air. This means that an organism that lives in water will have to apply a greater force to move forward.
The ability to resist the movement of any object moving in any media (other than vacuum) is known as friction.
In denser and viscous mediums, friction is greater. So, the fishes have evolved to develop a streamlined body that allows easy movement of water across its body which reduces friction significantly.

Note:
Most of the aquatic organisms have radial symmetry. Animals with radial symmetry have a cylindrical axis around which the body organs are arranged. This cylindrical axis offers the least resistance (friction) to water and allows better movement in the aquatic environment. Bilateral symmetry also helps in aquatic environments but is less common.
The situation is just the opposite in terrestrial habitats. The animals of terrestrial environments, especially those which can move, have bilateral symmetry which balances the body weight on both the sides of the animal as it moves and thus, offers stability to the body when it is in a dynamic state.