Why does fresh water fish has no scale compare to sea fish?

It's not true that freshwater fish have no scales compared to sea fish. Both freshwater and sea fish have scales, but there are some differences in the structure and composition of their scales. Here's a comparison:

Freshwater Fish Scales:

- Type of Scales: Most freshwater fish have cycloid scales, characterized by a smooth and rounded shape.

- Composition: Cycloid scales are primarily made of a protein called collagen and have a thin layer of enamel covering them.

- Flexibility: Cycloid scales are flexible and embedded in the fish's skin. They allow for some flexibility in body movement.

Sea Fish Scales:

- Type of Scales: Sea fish can have different scale types, including ctenoid scales, cycloid scales (similar to freshwater fish), or ganoid scales (found in certain ancient and primitive fish species).

- Ctenoid Scales: Ctenoid scales have a ctenii structure, which means they have comb-like or serrated edges. This gives them a rougher texture compared to cycloid scales.

- Composition: Like freshwater fish, the primary component of sea fish scales is collagen, but the enamel layer is thicker and harder.

- Strength and Protection: Ctenoid and ganoid scales provide enhanced strength and protection for the fish in various marine environments.

So, while both freshwater and sea fish have scales, the key differences lie in the type of scales and their structural characteristics. Sea fish may have stronger and thicker scales to withstand the rigors of marine environments.