Short-term responses of adult kuruma shrimp Marsupenaeus japonicus (Bate) to environmental salinity: osmotic regulation, oxygen consumption and ammonia excretion

The effects of salinity on haemolymph osmolality, oxygen consumption and ammonia excretion were investigated in adult kuruma shrimp, Marsupenaeus japonicus (Bate), at salinities of 20, 25, 30 and 35 g L^-1. Haemolymph osmolality showed a positive linear relationship (r^2=0.9854) with medium osmolality. The isosmotic point calculated from this relationship was 1039 mOsm kg-1, which corresponds to a salinity of approximately 35 g L^-1. The slope of the regression equation was very high (0.81), suggesting that M. japonicus adults are poor osmoregulators compared with the adults of other penaeids and to conspecific young. The difference between haemolymph and medium osmolality (DOP) was lowest at 35 g L^-1 and highest at 20 g L^-1. Thus, the minimum DOP coincided with the isosmotic point of the shrimp. The respiration rate was significantly lower at 30 g L^-1 than at the other salinities. Ammonia excretion rates were inversely related with salinity and therefore were minimal at 35 g L^-1. The results of this study suggest that the optimum salinity for adult kuruma shrimp is around 30-35 g L^-1 and that even minor (e.g. 5 g L^-1) deviations from this optimum cause significant physiological changes. Further, the observed increases in oxygen consumption and ammonia excretion during exposure to low salinities, which indicate higher energy expenditure and amino acid catabolism for osmoregulation, respectively, suggest that the growth efficiency of M. japonicus adults may be severely compromised by hypohaline water inflow into the rearing ponds.