Ocean Acidification

Highlights

• Acute acid stress (pH 4.5) preferentially impairs later mating stages in medaka, rather than causing a generalized reproductive collapse.
• Early initiation phases (following and courtship) are relatively less affected, whereas progression to the crossing stage and spawning is strongly suppressed.
• General locomotion and territorial aggression show no detectable change under acute acidification.
• Females exhibit increased ammonia excretion and oxygen consumption, indicating sex-specific physiological strain under acid stress.
• The behavioral impairment is consistent with an energy reallocation toward acid–base homeostasis, reducing investment in energetically costly terminal mating behaviors.

Abstract

Environmental acidification poses a significant threat to aquatic organisms, yet the underlying mechanisms of how acid stress disrupts complex social and reproductive behaviors remain incompletely understood. In the present study, we investigated the behavioral and physiological responses of adult Japanese medaka (Oryzias latipes) under acute acid exposure (pH 4.5, 24 h). Our results showed that acute acid stress reduced mating success, with a stronger impairment observed in the transition to the crossing stage and subsequent spawning, while the earlier, male-driven stages of following and courtship were relatively less affected under the present experimental conditions. This reproductive impairment did not coincide with detectable changes in generalized emotional-like behavior. Fish showed normal performance in the novel tank test and maintained territorial aggression. Physiologically, we observed a clear sexual dimorphism. Under acid stress, males maintained relatively stable MO₂ and J_Amm, whereas females showed elevated MO₂ and J_Amm. These findings suggest that reproductive impairment may be more pronounced in energetically demanding mating stages and may be associated with sex-dependent physiological strain. We further propose that the increased metabolic cost of acid-base regulation in females could shift energy allocation, and that high-cost coordination behaviors may be reduced in order to maintain short-term survival. This study provides evidence linking bioenergetics, acid-base regulation, and reproductive behavior in a teleost model under acidification.

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 (Oryzias melastigma) 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.