Low dissolved oxygen levels increase stress in piava (Megaleporinus obtusidens): iono-regulatory, metabolic and oxidative responses

High dissolved oxygen exacerbates ammonia toxicity with sex-dependent manner in zebrafish

Ammonia nitrogen is one of the important environmental factors, and causes negative effects for fish health in ecosystem and aquaculture. The toxic effects and mechanisms of ammonia in fish deserve further investigation. In the present study, we exposed female and male zebrafish (Danio rerio) to ammonia (50 mg/L NH₄Cl) with oxygenated (7.5-7.8 mg/L) or non‑oxygenated (3.8-4.5 mg/L) water, to identify the combined effects of dissolved oxygen and ammonia on fish with gender difference. The results showed that oxygenated ammonia exposure increased fish mortality, gill secondary lamellas damage and gill tissue spaces, gene expressions of proinflammatory interleukin 1 beta (il-1β) and apoptotic caspase8 as compared with non‑oxygenated ammonia. Besides, oxygenated ammonia elevated plasma ammonia contents, and decreased the discharge of body ammonia through gills by depressing the enzyme activity of Na⁺/K⁺-ATPase. Notably, when zebrafish were subjected to ammonia stress, more severe mortality, gill damage and tissue inflammatory response were observed in males than females. This is the first study to clarify the gender-dependent impacts of ammonia toxicity, and the adverse effects of oxygenation on ammonia resistance in zebrafish.

Temporal variations in glucose level, oxidative stress and antioxidant profile of Koi carps (Cyprinus carpio var koi) in response to hypoxic tolerance

Biochemical analyses were conducted temporally to understand the impact of prolonged hypoxic environment on gill and white muscle of juvenile Koi carps (Cyprinus carpio var koi). The 16-day study involved quantification of Glucose (GLU), oxidative stress (Lipid Peroxidation-LPO) and antioxidant profile (Catalase-CAT, Glutathione-GSH, Glutathione-S-Transferase-GST) every 96th hour to understand the physiological consequences of hypoxic environment compared to normoxia (Control). There was no mortality throughout the study period, however, fishes were physically exhausted and exhibited Aquatic Surface Respiration (ASR) after day 8. Reduction in glucose levels was observed for both the tissues from day 4 to 16. LPO values increased in gill from day 4 to 12 followed by decrease on day 16. Catalase values reduced in gill from day 4 to 12 with an abrupt spike on day 16. Among the glutathione antioxidants, negligible GST activity was observed for day 12 and 16 in gill and muscle, while day 4 and 8 revealed fluctuations in GSH for both the tissues. The study showed tissue-specific differential response to stress because of a variance in the evaluated biomarkers in gill and muscle. Understanding the extent of physiological response to progressive oxygen disruption is essential for best practices in pisciculture because its availability is a critical factor impacting fish health which is congruent to the results of this study.