Air-breathing fishes: Metabolic biochemistry of the first diving vertebrates

Two main legacies of air breathing in mammals and birds are endothermy and high-energy flux life styles, the latter of course allowing the former. While these two characteristics are also achievable in some water breathers (tuna, swordfishes, some sharks), they are generally absent in other fishes. As in the vertebrate lineage, air breathing was first invented by fishes, it is appropriate at the outset to emphasize that endothermy and high-energy flux life styles are not key outcomes of air breathing in fishes. In contrast, air-breathing fishes typically display low energy fluxes and are relatively sluggish animals. To appreciate why this is so, it is important to recall the kinds of selective forces that led to the evolution of air breathing in fishes in the first place. The chapter discusses that air breathing is an ancient development in many tropical fishes; as its acquisition appears to be less a matter of sustaining high-energy fluxes than a matter of simply surviving in poor oxygen conditions, one may appropriately look for a metabolic biochemistry that evolved accordingly. © 1995, Elsevier B.V. All rights reserve