Mechanisms of gill ventilation.

Some species use both buccal pumping and ram ventilation, while some can only use one. (a) Buccal pumping. The expansion of the buccal and opercular cavities acts like a suction pump, and closing these cavities acts like a pressure pump. During this process, flaps of tissue in the mouth, throat, and operculum help prevent water from moving in the reverse direction. (b) Ram ventilation. Many fish, such as this tuna, can ventilate their gills by swimming forward or facing upstream in moving water with their mouths open. This causes the water to move through the mouth and operculum.