Marine aquatic ecosystems rely heavily on dissolved oxygen (DO) as the lifeblood sustaining their inhabitants. Within these environments, dissolved oxygen serves as the critical respiratory substrate for diverse organisms, including turtles and fish. These animals extract dissolved oxygen directly from surrounding waters through specialized respiratory structures—fish via gill filaments and turtles via pulmonary systems adapted for prolonged submersion. The metabolic demands of their tissues drive the diffusion of dissolved oxygen across respiratory membranes, fueling aerobic processes essential for survival, growth, and reproduction. For ectothermic marine reptiles such as sea turtles, efficient extraction of dissolved oxygen becomes paramount during prolonged dives, where breath-holding capacities are stretched to limits. Similarly, teleost fish face challenges in maintaining dissolved oxygen uptake across gill surfaces as environmental temperatures rise or pollution reduces oxygen solubility in water. The balance of dissolved oxygen thus emerges as a pivotal determinant of species distribution, trophic interactions, and ecosystem resilience in marine systems. Understanding these dynamics is crucial for conserving vulnerable species and mitigating anthropogenic impacts on aquatic life.