Respiratory plasticity of red drum to chronic hypoxia

Ocean deoxygenation (hypoxia) is a pressing concern in the face of climate change as hypoxic areas increase in size, duration, and magnitude with each year. When a fish cannot escape hypoxia, it must be able to make the appropriate physiological adjustments to maintain fitness. Juveniles and adults can display reversible phenotypic changes in low oxygen (O₂), while embryos and larvae may producing fixed traits that are carried to adulthood through developmental plasticity. Across all life stages aerobic metabolism is the most efficient way that fish generate energy, and the most impacted pathway under hypoxia. In my dissertation, I explore the flexible responses that compensate for changes to aerobic metabolism across different life stages of the marine teleost red drum (Sciaenops ocellatus) exposed to chronic, sub-lethal hypoxia. Hypoxia-acclimated juvenile drum demonstrated significant changes in hemoglobin (Hb) isoform expression relative to control. Changes in Hb expression co-occurred with reduced pH sensitivity, and increased O₂ binding affinity. Additionally, this correlated with increased maximum metabolic rate and aerobic scope relative to controls in hypoxia. These results demonstrate an important role for Hb isoforms in maximizing respiratory performance in hypoxia with implications at the whole-animal level. Furthermore, I investigated how acclimated fish respond to exhaustive exercise, and their anaerobic swim performance. I found that hypoxia-acclimated juveniles decreased ATP in the red muscle and increased ATP and glycolytic potential in their white muscle. This phenotype recruited white muscle at lower swim speeds than control fish, indicating a prioritization of glycolytic white muscle swimming over aerobic red muscle. Finally, I assess whole animal respiratory and swim performance in fish exposed to hypoxia during a critical window in early development. These fish show increases in aerobic performance in normoxia, while becoming more vulnerable to hypoxia in later life. I sought to understand the mechanisms and implications of hypoxia-induced respiratory adaptations, and demonstrated the ability of a marine fish to adapt to environmental stress, and how these adaptations changed in different life stages. This work demonstrates the species-specific resiliency and limitations in environmental stress, and illustrates the need for more species-specific work in a changing ocean.