INTERACTION OF IONIZED AND UN-IONIZED AMMONIA ON SHORT-TERM SURVIVAL AND GROWTH OF PRAWN LARVAE, MACROBRACHIUM ROSENBERGH

Abstract

The sensitivity of larval shrimp to ammonia was studied in a series of static toxicity experiments using M. rosenbergii. Proportions of NH3 and .**GRAPHIC**. were varied by using pH values of 6.8, 7.6 and 8.4 in an effort to compare the susceptibility of larvae to these two ammonia forms. Growth was monitored among larval groups exposed to sublethal concentrations of ammonia at these 3 pH values. The sensitivity of larvae of NH3 was significantly influenced by pH; the 144 h LC50 values were 80, 44 and 14 mg NH3/l at respective pH of 6.8, 7.6 and 8.4. Toxicity was not due solely to the NH3 molecule. In solutions of different pH and equal NH3 concentrations, survival was greatly reduced as .**GRAPHIC**. levels increased. A model to explain the differential effects of NH3 as pH varied proposes that toxicity results from copious diffusion of NH3 into larvae at high pH (8.4). At low pH (6.8) toxicity may result from competitive inhibition of Na+-transport by .**GRAPHIC**. The average dry weight of larvae exposed to sublethal NH3 concentrations at pH 6.8 and 7.6 was 26% less than that of controls after a 7 day exposure.