Gill surface area must be large enough to provide adequate exchange of gases. Therefore, highly active fish have largest relative gill area. Figure compares highly active fish with sluggish bottom dwellers. Their relative surface area is also given.
Figure: Fast swimming fish like makerel have larger gill area than sluggish bottom dwelling fish. The total surface area of gills is expressed per unit per gram body weight of fish.
For gas exchange to be adequate there should be adequate flow and close contact between the gill and water. To understand how this happen let us look at the structure of a gill in bony fishes as a representative of an aquatic respiratory surface. Gills are enclosed in a gill cavity. This provides protection for the fragile organ and also permits the water to run over the gills in an efficient manner. Gills of fishes consist of several gill arches on either side. The gill arches separate the opercular and P the buccal cavities. From each gill arch extend two rows of gill filaments.