In aquaculture systems, ammonia nitrogen is a major component of waste products arising from the metabolism of cultured organisms and can significantly affect the quality of the rearing environment. The purpose of the current study was to determine modulation of the haemocyte population and selected haemolymph biochemical parameters of the Mediterranean mussel Mytilus galloprovincialis (Lamarck, 1919) exposed to acutely elevated ammonia concentrations.In order to investigate the extent and dynamics of an expected reduction of physiological fitness and survival rate of cultured mussels the stress-on-stress test was conducted combining ammonia and air exposure. The latter test illustrated the extent of reduced mussel physiological fitness and survival rate upon 72 h exposure to 10 and 30 mM NH4Cl in experimental tanks followed by additional exposure to air. Mussels cultured for up to 48 h in experimental tanks comprising 5, 10 and 30 mM NH4Cl, revealed dose and time dependent increase in haemolymph enzyme lactate dehydrogenase levels, urea, and uric acid metabolite levels, as well as the corresponding redistribution in haemocyte population profiles, indicating an inherent response of this organism to ammonia. Finally, a significant negative linear correlation between the mussel hyalinocytes subpopulation and the mean urea levels was observed. The obtained result suggests a possible function of basophilic and acidophilic granulocyte subpopulations in ammonia metabolism leading to urea synthesis by means of surplus ammonium ions are sequestered and extracted from the body. Therefore, defined modulation of the mussel haemocyte population structure may serve as an accurate bioindicator of mussels exposure to acutely elevated ammonia concentrations in environmental and aquaculture biomonitoring studies.
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