Q24. Why do fish open and close the mouth and operculum during respiration?

The fish opens its mouth to take in water inside. When a fish opens and closes its mouth, it is actually pumping water back through the gills and is thus breathing. Operculum is a hard plate that covers the gills on each side of the fish head. The movements of opercula put water over the gills, which enable the fish to take oxygen. When the fish mouth is open, water enters and the opercula close over the gills. When the mouth closes, the opercula open allowing oxygenated water to flow over the gills and to force water outside the fish.