Air-breathing Fishes Fishes that occasionally gulp atmospheric
air, such as lungfishes, are no different from other
fishes when they are actively water breathing. They use the
same dual pump mechanism to irrigate their gills. However,
when the lungfish breathes air, the dual pump is modified
into a buccal pump to move air in and out of the lungs. The
four-stroke buccal pump can be summarized as an exhalation
phase and an inhalation phase. The exhalation phase begins
with the transfer (expansion 1) of spent air from the lungs
into the buccal cavity. In some fishes, relaxation of a sphincter
around the glottis permits this transfer from the lungs to
the buccal cavity. Exhalation concludes with expulsion
(compression 2) of air from the buccal cavity to the outside
either through the mouth or under the operculum. As the
fish rises and its snout breaks the surface, its mouth opens to
intake (expansion 3) atmospheric air, the first step in the
inhalation phase. Inhalation concludes with compression (4),
which forces a bubble of fresh air from the buccal cavity into
the lungs (figure 11.12).