The structure and function of fish.

The body of fish has a streamlined body shape, allowing to reduce water resistance, mucus secreted by the skin glands reduces friction. Fish breathe through the gills, and the nostrils provide the perception of the smell of substances dissolved in the water.
The presence of the sideline allows the fish to determine the direction and strength of the current.
The swim bladder allows the fish to sink and float.
Functions of fish in the ecosystem: they are food for other larger predatory fish and humans, they regulate the number of organisms that they eat and some are orderlies (sharks), destroying the weak and sick individuals.