Breathing Underwater: From Oxygen Sensing to Behavior in Larval Zebrafish

Abstract

Oxygen (O₂) is essential for nearly all animal life, serving as the final electron acceptor in the mitochondrial electron transport chain and enabling the efficient generation of ATP to meet the energetic demands of multicellular organisms. However, despite its abundance in the atmosphere, oxygen availability is often unpredictable in aquatic environments. Many organisms, particularly aquatic vertebrates, must contend with acute drops in environmental oxygen or chronic hypoxic conditions during development. Understanding how vertebrates sense, respond to, and survive fluctuations in oxygen levels remains an important area of biological inquiry. Larval zebrafish (Danio rerio), with their optical transparency, genetic tractability, and quantifiable behaviors, provide a powerful model to investigate the neural circuits and behavioral strategies underlying hypoxia adaptation. This dissertation explores how larval zebrafish behaviorally and physiologically respond to hypoxia, employing a multidisciplinary approach that integrates behavioral assays, in vivo calcium imaging, laser ablation techniques, and serial electron microscopy-based circuit reconstruction. The studies presented here reveal that acute hypoxia triggers a robust and stereotyped motor behavior characterized by rhythmic pectoral fin movements. These movements likely serve an adaptive role by enhancing water flow across respiratory surfaces, thereby facilitating oxygen uptake. Calcium imaging experiments suggested specific motor neurons whose activity correlates with hypoxia-induced fin movements. Laser ablation of suggested nerve bundles confirmed the neural pathways controlling this behavior. Beyond acute responses, this dissertation examines the developmental consequences of chronic hypoxia exposure. Larval zebrafish raised under sustained low oxygen conditions exhibited altered growth patterns, including slow growth along the anteroposterior axis and a delayed onset of swim bladder inflation. Behavioral assays assessing the optomotor response revealed that chronic hypoxia impairs sensorimotor coordination during critical stages of development, potentially affecting the larvae’s ability to navigate and forage effectively. These findings suggest that oxygen availability during early development has lasting impacts on morphology and neural circuit function. Together, these studies illuminate the strategies by which vertebrates transform oxygen-sensing into coordinated motor responses and offer a foundation for further investigations into the strategies for counteracting hypoxia, the interplay between environmental stress and nervous system development, and potential translational relevance for understanding hypoxia-related challenges in human health.