Question

Why does a fish open and close its mouth and operculum during respiration?

Answer 1

the fish opens its mouth to intake water so that it can intake oxygen dissolved in water

Answer 2

Fish open and closes it's mouth to intake water and send it to the gills which helps in the absorption of the essential nutrients and oxygen to the different body parts. Operculum is a hard plate that covers the gills on each side of the fish head. To pump out water through the gills fish had to open and close the operculum so they can breath in the water.