Mucus 

Silver Dollars,  Photo by Pete

Fish have special cells in their skin which constantly secrete mucus. This mucus layer completely covers the fish including the fins. It is made up of many chemicals and it acts to stop bacteria and other infections and parasites which it is able to trap and kill in healthy fish this layer also prevents fungus from gaining a foot hold. This is why a careful eye must be kept on what might appear to be trivial injuries at first. It also acts as a lubricant and helps the fish to slide through the water with less drag which save energy when swimming.

In some other fish it is the Fry's first food, (Discus and some Catfish). And contains substance to promote the development of the Fry's immune system. Like colostrum in mammals.

Discus fry feeding on mucus. Photo by: Andy

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