Ammonia Toxicity in Bivalve Molluscs: A Review

Ammonia is a major toxicant of concern in aquaculture systems, originating from anthropogenic inputs such as excessive feeding, sewage discharge, agricultural runoff, and industrial effluents. In aquatic environments, ammonia exists as ionized ammonium (NH₄⁺) and un-ionized ammonia (NH₃), with the latter being highly toxic to aquatic organisms. Bivalve molluscs are particularly susceptible to ammonia toxicity due to their filter-feeding behaviour and continuous exposure to ambient water. Elevated ammonia concentrations adversely affect growth, feeding, behaviour, and immune responses in bivalves, leading to oxidative stress, tissue damage, and metabolic dysfunction. Behavioural responses such as prolonged shell opening and reduced foot retraction are commonly observed indicators of physiological stress. In addition, ammonia exposure causes marked alterations in haemocyte counts, serum glucose, total protein levels, and hepatic enzyme activities, including AST, ALT, and ALP, reflecting metabolic and immunological imbalance. This review critically synthesizes current knowledge on ammonia toxicity in bivalve molluscs, highlighting physiological, biochemical, and immunological responses, and underscores the importance of effective ammonia management for sustainable bivalve aquaculture. In the context of increasing environmental fluctuations, advancing our understanding of ammonia-mediated stress responses is crucial for safeguarding bivalve populations and ensuring the resilience of aquatic ecosystems.