The evolutionary ecology of respiratory mode in fishes: an analysis based on the costs of breathing

Abstract

Fishes use unimodal water breathing, unimodal air breathing, and a wide range of bimodal combinations of water and air breathing to obtain oxygen. This essay seeks to provide a theoretical framework in which to understand this diversity of respiratory mode. Consideration of oxygen as a resource shows that it is scarce in relation to demand and that an inadequate supply limits activity, growth, reproduction, and ultimately, survival. Thus, there should be strong selective pressure to maximize the efficiency of oxygen uptake. Both water breathing and air breathing require ventilation, circulation, locomotion, and respiratory structures. Each of these components has energy costs and may also be subject to costs in time, materials, or risk of predation. Consideration of these costs reveals that dissolved oxygen concentration and distance from the surface are key environmental determinants. Predation pressure and several morphological and behavioral characteristics may also have an influence. These factors are integrated into a general theory of breathing costs which permits prediction of patterns in respiratory partitioning, habitat selection, the frequency of occurrence of respiratory modes in different habitats and correlations within habitats between behavioral and morphological characteristics and respiratory mode.