Most fish are covered with hard scales, but a few fish, such as eel, whale and mud, are covered with sticky liquid. This is because the scales on their bodies have degenerated, and there are many special mucus glands directly exposed to the outside skin, which can secrete a large amount of mucus and form a mucus layer. Fish scales have a protective effect on fish, and mucus has a similar function. Although it can not block the impact of hard objects, it can prevent the invasion of poisonous bacteria and prevent harmful substances in water from entering the body from the skin. In fact, the role of mucus is far more than that. With its existence, the fish’s skin can be impervious, which is good for maintaining the constant osmotic pressure in the fish. In particular, some fish swimming in rivers have mucus, which can make them adapt to the changes of salt concentration in the water, reduce the friction of water and help the fish swim faster and more labor-saving. Therefore, mucus is an indispensable magic weapon for fish survival.