the lung surface area of a terrestrial mammal (Fig. 2). This is un
doubtedly related to the high levels of sustained activity that can be
maintained by tuna. In general, however, the area of the respiratory
surface is smaller in fish than in terrestrial mammals. This may be related
to either the lower rates of oxygen consumption in fish or the dual func
tion of the gills. The gills are an important site for ion and water exchange
as well as gas exchange (see chapter by Conte, Volume I). An increase
in the size of the gills increases ion and water exchange as well as gas
exchange. Fish in fres hwater may have to restrict the surface area their gills in order to reduce ion and water exchange, and it is possible
that the large surface area of the gills of tuna could only have evolved
under the reduced osmotic load of seawater.
The surface area of the gills tends to be reduced in air-breathing
fish (Dubale, 1951 ). In these fish the secondary lamellae are often far
apart, and unlike non-air-breathing fish, the lamellae do not collapse
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