Identification of fish

Many people have misunderstandings about the concept of fish. When they see an animal that lives in water and looks like a fish or has the word "fish" in its name, they think it belongs to a fish. For example, whales (mammals), cuttlefish (mollusks), abalone (mollusks), soft-shelled turtle (reptiles), giant salamander (amphibians), crocodile (reptiles) and so on are often wronged by this misunderstanding. Generally speaking, an animal must meet four conditions to be considered a fish:

First, it must be a vertebrate that lives in water all its life; Some fish can stay on land for a short time.

Second, swim (exercise) through the swing of tail and trunk and the coordination of fins.

Third, breathing mainly depends on the gills, and a small part can also breathe with the lungs, such as lungfish.

Fourth, the body surface is often covered with scales.