Physiology of Finfish and Shellfish (2+1)

2.1.14. Gill area and diffusion capacity

The gill area of a fish depends on the number and size of the gill filaments and gill lamellae. It has been shown that fast swimming and active fishes possess more gill area and higher number of gill lamellae/mm of gill filament as compared to sedentary species. Air breathing fishes, mainly have accessory respiratory organs, in gill area reported to be ½ of the area of fishes that depends on aquatic respiration. The total gill area increases with the increase in body weight as the fish grows. The efficiency of the gill as a respiratory organ depends on its oxygen diffusion capacity in water, which is inversely proportional to the blood water barrier (diffusion distance) i.e., the barrier through which the exchange of gases takes place between blood and water. This barrier is formed by the lamellar epithelium, basement membrane and the flange of the pillar cells. (The pillar cells or pilaster cell are a characteristic feature of the teleost gill and serve as pillars to keep the 2 epithelial layers of the lamella separate and prevent them from collapsing). The thickness of the barrier is less in species depending on water for respiration than in air breathing fishes. The teleostean gill is capable of diffusing 60-80% of oxygen from water to blood at the gills.