Histological, physiological and transcriptomic responses of the gill, kidney and liver of orange-spotted grouper (Epinephelus coioides) to acute ammonia stress

Ammonia is a pervasive environmental stressor in intensive aquaculture systems that compromises fish health and productivity. Here, we characterized histological, physiological, and transcriptomic responses in the gill, kidney, and liver of surviving Epinephelus coioides following initiation of acute ammonia exposure (31–32 mg/L TAN; estimated 1.13–1.80 mg/L NH 3 -N) for up to 72 h. Ammonia exposure induced pronounced organ-specific dysfunction and temporally dynamic responses. Histologically, the gill progressed from lamellar thickening and chloride cell hypertrophy (12 h) to severe lamellar fusion and edema (72 h). In the kidney, glomerular atrophy was persistent, whereas renal tubular lumen diameter increased at 12 h and returned to near-baseline values by 72 h. The liver showed progressive vascular congestion and hepatocellular distortion. Physiologically, branchial NKA was suppressed throughout the exposure, while SOD activity decreased significantly at 12 h. Conversely, the kidney exhibited consistently elevated SOD and CAT at both time points, accompanied by a transient spike in NKA at 12 h and a delayed upregulation of GST at 72 h. Hepatic SOD and CAT activities increased at 12 h; by 72 h, SOD decreased below the baseline level, whereas CAT returned approximately to baseline. Transcriptomic analyses identified tissue-associated co-expression modules and mmp13 as a commonly dysregulated candidate gene. Overall, surviving fish showed concurrent but organ-specific responses to acute ammonia exposure.