Low-Oxygen Lifestyles

Aquatic hypoxia (low oxygen) provides a useful system for exploring ecological and evolutionary consequences of living under extreme conditions. It is also an environmental stressor of accelerating interest due to human activities that have increased the extent of hypoxic waters on a global scale. This chapter characterizes the distribution of hypoxic habitats, reviews key adaptations of fishes to extreme hypoxia, and explores the role of hypoxia as a divergent selective factor. Trade-offs in the costs and benefits of living in hypoxic and normoxic habitats may contribute to faunal diversification by creating spatially divergent selection that leads to specialized phenotypes as illustrated in studies of African fishes from hypoxic swamps and associated normoxic sites. In these systems alternative dissolved oxygen (DO) environments provide a strong predictor of intraspecific variation, particularly in traits related to oxygen uptake efficiency or oxygen limitations, but also in characteristics indirectly affected through trait correlations. Studies of fish persisting under hypoxia highlight the importance of localized extreme habitats as model systems for studying divergent natural selection and more generally for exploring effects of physicochemical stressors on ecological and evolutionary processes.