Fish posses well developed gills for extraction of oxygen from the medium.
The surface of the gills are moist
They have a larger surface area to extract more oxygen
They are transparent for oxygen and thin so oxygen can penetrate easily.
A concentration gradient is maintained to extract oxygen throughout the circulatory system.
The respiratory surface is ventilated by having a continuous flow of water over the gill surface using the mouth as a pump.
In cartilage fish the gills open separately as gill slits while in bony fish the gills are covered by an operculum. The operculum functions as an additional pump to help the buccal pump.