An integrative analysis of respiratory capacity, osmoregulatory function, body size, and metabolism in intertidal fishes

Ocean warming and eutrophication-driven hypoxia are two key threats marine organisms face with climate change and human activities. The impacts of warming and hypoxia on aerobic metabolism and respiratory capacity are thought to be a key mechanism underlying the susceptibility of marine organisms to these stressors, but how evolution and physiological tradeoffs affect these traits remains an area of intense investigation. Using intertidal sculpin fishes (Actinopterygii: Cottidae) as a model system, I studied how adaptation to the temperature- and oxygen-variable intertidal has shaped the temperature sensitivity of hypoxic respiratory capacity, a trait thought to shape biogeographic ranges in marine organisms broadly. Using the critical oxygen tension for standard metabolic rate (Pcrit) as a measure of hypoxic respiratory capacity, I found that tidepool-specializing species maintain greater capacity for oxygen uptake (i.e., lowest Pcrit) in warm, hypoxic water compared to subtidal congeneric species. Respiratory capacity and osmoregulation are linked in water breathing organisms through the osmorespiratory compromise, so I assessed whether improved respiratory capacity came with an osmoregulatory performance penalty in the tidepool-specialist Oligocottus maculosus. Based on diffusive water flux, an index of water permeability and a key component of osmoregulation, I found that O. maculosus suppressed water permeability in hypoxia but this suppression was lost during combined exposure to hypoxia and high temperature. These results suggest that the impacts of single vs. multistressor conditions may impact metabolic scope for ecologically-relevant activities in complex, difficult to predict ways. Lastly, based on the resurgent but controversial gill oxygen limitation hypothesis (GOLH), I analyzed whether gill surface area underlies the relationship between body size and respiratory capacity in O. maculosus. I did not find support for GOLH in O. maculosus, but hypermetric scaling of ventricle mass may contribute to the hypermetric scaling of maximum oxygen uptake rate in this species. Together my results suggest increased hypoxic respiratory capacity may be a potential evolutionary response to combined warming and hypoxia. An important avenue for further investigation is the impact of the interactions between respiratory capacity and linked processes such as osmoregulation found here on key ecologically-relevant performances such as growth and reproduction.Science, Faculty ofZoology, Department ofGraduat