SKIN-RESPONSES AND MORTALITIES IN THE LARVAE OF JAPANESE CROAKER EXPOSED TO AMMONIA

One day old larvae of the Japanese croaker Nibea japonica were exposed to low levels of ammonia, at concentrations of 0.0027, and 0.027 mg/m l NH3-N (un-ionized ammonia) for 72 h. High mortalities were observed in the groups exposed to 0.0027 and 0.027 mg/l NH3-N. Because these am monia levels are frequently found in the nursery ponds of various spec ies, the acceptable ammonia concentrations for larval culture must be reevaluated. In the second experiment, 3-day-old N. japonica larvae we re exposed to a high level of ammonia (0.26 mg/l NH3-N). Pathological changes were observed in the skin by light and electron microscopy. Th e changes included reduction in the number of mucous cells, cytoplasmi c vacuolation, rough apical surface of the epithelial cells and degene ration of the mitochondria of the chloride cells.