Survival, osmoregulation and ammonia-N excretion of blue swimmer crab, Portunus pelagicus, juveniles exposed to different ammonia-N and salinity combinations.

Ammonia-N toxicity to early Portunus pelagicus juveniles at different salinities was investigated along with changes to haemolymph osmolality, Na(+), K(+), Ca(2+) and ammonia-N levels, ammonia-N excretion and gill Na(+)/K(+)-ATPase activity. Experimental crabs were acclimated to salinities 15, 30 and 45 per thousand for one week and 25 replicate crabs were subsequently exposed to 0, 20, 40, 60, 80, 100 and 120 mg L(-1) ammonia-N for 96-h, respectively. High ammonia-N concentrations were used to determine LC(50) values while physiological measurements were conducted at lower concentrations. When crabs were exposed to ammonia-N, anterior gill Na(+)/K(+)-ATPase activity significantly increased (p<0.05) at all salinities, while this only occurred on the posterior gills at 30 per thousand. For crabs exposed to 20 and 40 mg L(-1) ammonia-N, both posterior gill Na(+)/K(+)-ATPase activity and ammonia-N excretion were significantly higher at 15 per thousand than those at 45 per thousand. Despite this trend, the 96-h LC(50) value at 15 per thousand (43.4 mg L(-1)) was significantly lower (p<0.05) than at both 30 per thousand and 45 per thousand (65.8 and 75.2 mg L(-1), respectively). This may be due to significantly higher (p<0.05) haemolymph ammonia-N levels of crabs at low salinities and may similarly explain the general ammonia-N toxicity pattern to other crustacean species.