When you go swimming in the river, you are told to be careful about the rocks, as you may bruise yourself if you hit against them.
But, fish swim in the water all the time. Through cracks and crevices in rocks inside the water, and between the branches of thorny water plants. They do not get bruised so easily.
And its not because their parents had given them sound advice. They don’t have to. They know the little fish are safe.
Fish have tough scales on their bodies which serve as a protective layer and guard them against predators and a harsh environment.
The toughness of the scales depends on the lifestyle of the fish and its home. That is why the scales of fishes who live near the coast or in reefs are always stronger than those of fishes living in deeper waters, where they do not encounter too many ragged surfaces.
For instance, the scales of the Coelacanth fish are so rough and tough that people living in the Comoro Islands use it as a substitute for sandpaper.
Fish scales are also covered with mucus that makes the skin slippery and gives extra protection. The mucus is produced by special cells under the scales.