Importance of balanced Na+/K+ ratios for blue swimmer crabs, Portunus pelagicus, to cope with elevated ammonia-N and differences between in vitro and in vivo gill Na+/K+-ATPase responses

Abstract

It is well known that altered Na+/K+ ratios of water can reduce the survival and growth of crustaceans due to osmoregulatory disruptions. However, since the mechanisms involved with osmoregulation are also linked with ammonia-N excretion, it is unclear whether altered Na+/K+ ratios will also affect the ability of crustaceans to withstand ammonia-N exposure. To investigate this the ammonia-N toxicity to early blue swimmer crab, Portunus pelagicus, juveniles was measured at low (18.2), normal (27.3) and high (54.6) Na+/K+ ratios with ammonia-N concentrations set at 0, 1.42, 2.85, 4.28, 5.71 and 7.13 mM (20 replicate crabs/treatment). The crabs were exposed to these combinations for 96-h and at the end of the experiment the hemolymph osmolality, hemolymph Na+, K+, Ca2+ and ammonia-N levels, gill Na+/K+-ATPase activity and gill histopathology were measured. Ammonia-N excretion was also measured at 72-h and 96-h. The results revealed that under both low and high Na+/K+ ratios the ammonia-N toxicity to P. pelagicus juveniles significantly increased. This was likely linked with a significant decrease to gill Na+/K+-ATPase activity and ammonia-N excretion rates along with significantly higher hemolymph ammonia-N levels and greater gill histopathological damage. A subsequent in vitro experiment measured gill Na+/K+-ATPase activity of P. pelagicus under various ammonia-N (0, 3.56 and 7.13 mM) and K+ (25, 50 and 75 mM) combinations. Interestingly, gill Na+/K+-ATPase activity of the crabs was not significantly affected by either ammonia-N or K+ and no significant interactive effect between the two was detected. The toxicity pattern at sub-optimal Na+/K+ ratios should be noted by aquaculturists utilizing inland saline water and, furthermore, it is suggested that future in vitro studies may also need to consider incorporating in vivo tests to evaluate their applicability to living organisms.