Mechanisms, evolution, and phenotypic plasticity of hypoxia tolerance among bass and sunfish (family Centrarchidae)

Abstract

Hypoxia events are increasing in severity, frequency, and duration in aquatic systems as pressure from pollution and global warming continues to accumulate. Many studies have examined hypoxia tolerance in fish, but few have determined the relationship that hypoxia tolerance has with other organismal traits, such as exercise performance or thermal tolerance. In this thesis, I used fish from the family Centrarchidae to examine the potential tradeoff between hypoxia tolerance and exercise performance, and to discover the physiological determinants of oxygen transport that underlie these traits. I found evidence for a tradeoff between hypoxia tolerance and exercise performance, with largemouth bass having the best exercise performance but the poorest hypoxia tolerance. Rock bass had the best tolerance of hypoxia but the worst exercise performance, while pumpkinseed sunfish and bluegill sunfish were intermediate in both traits. Hypoxia tolerance and exercise performance were each underlain by some distinct traits, with increased swimming muscle capillarity conveying high performance and the capacity for anaerobic metabolism in the heart granting augmented hypoxia tolerance. However, some common traits defined both the most hypoxia tolerant and the best swimming species, including total surface area of the gills and gluconeogenic capacity in the liver. I also examined the effect that both temperature and acclimatization to the native environment had on hypoxia tolerance in sunfish. Increased temperature decreased hypoxia tolerance in all sunfish, and it had a greater effect on pumpkinseed sunfish than on the other species. Acclimatization to the native environment increased hypoxia tolerance compared to that in fish acclimated to the lab. This thesis advanced our understanding of how organismal traits can interact with one another and showed evidence for a tradeoff between hypoxia tolerance and exercise performance. In addition, I found that both exercise performance and hypoxia tolerance are underlain by some common and some distinct physiological traits. My work will contribute to our understanding of the effects of environmental stressors on performance and fitness in aquatic organisms