<p>Ocean acidification (OA), driven by atmospheric carbon dioxide (CO₂) emissions, significantly impacts marine organism. This study investigated OA effects on retinal structure and function in marine medaka (<i>Oryzias melastigma</i>) across life stages under controlled CO₂ exposures (450 × 10⁻⁶, 1 000 × 10⁻⁶, and 1 800 × 10⁻⁶). Quantitative analyses revealed concentration-dependent retinal pathology, with larval incidence rising from 16.7% (450 × 10⁻⁶) to 50.0% (1 800 × 10⁻⁶). Significant structural alterations include progressive pigment epithelium thickening and concurrent atrophy in inner plexiform and ganglion cell layers. Electrophysiological recording demonstrated severe neural hyperexcitability, with larval and adult fish showing 91.3% and 154.9% increased firing counts respectively at 1 800 × 10⁻⁶. Behavioral analyses revealed significant CO₂-induced locomotor dysfunction with distinct light-dependent patterns. Under illumination, larvae exhibited hyperactivity at 1 800 × 10⁻⁶, while adults showed progressive impairment (high-performance swimmers declining from 60% to 37.5%). Conversely, dark conditions intensified activity across concentrations, with adults displaying dose-dependent increases in swimming speed (23.5% to 68.8%). These results demonstrated that OA induces significant visual disruption and behavioral changes. The differential responses across life stages and light conditions suggest complex neurophysiological impacts that may have cascading ecological consequences for marine fish in acidifying ocean.</p>