Acute toxicity of ammonia and its effects on the haemolymph osmolality, ammonia-N, pH and ionic composition of early juvenile mud crabs, Scylla serrata (Forskål).

The current study was conducted to determine the LC(50) value of ammonia-N as well as the effects of acute exposure to elevated ammonia on the haemolymph osmolality, ionic composition, ammonia-N and pH levels of early juvenile mud crabs, Scylla serrata. The results show that early S. serrata juveniles have a high 96-h LC(50) value of 95.35 mg/L ammonia-N (6.81 mg/L NH(3)-N) or 6.80 mmol/L total ammonia-N (0.486 mmol/L NH(3)-N). Following a 96-h exposure, the haemolymph osmolality and K(+) levels of the surviving crabs remained unaltered (p>0.05) at all ammonia-N concentrations, while the haemolymph Na(+) and Ca(2+) were significantly lower (p<0.05) for the crabs exposed to 5.710 and 7.138 mmol/L ammonia-N. While the haemolymph ammonia-N levels of the crabs significantly increased (p<0.01) with increasing external ammonia-N concentrations, the haemolymph ammonia-N of the crabs remained below the external ammonia-N concentrations. The haemolymph pH of the crabs significantly increased between 0.714 and 4.283 mmol/L total ammonia-N. However, at 5.710 mmol/L total ammonia-N the haemolymph pH dropped and was not significantly different (p>0.05) from that of the control crabs which coincided with significantly lower (p<0.05) haemolymph Na(+) and Ca(2+) levels. These physiological responses may explain the high ammonia tolerance of early S. serrata juveniles.