Osmolality and ion balance in giant river prawn Macrobrachium rosenbergii subjected to changes in salinity: role of sex

Haemolymph osmolality, chloride and sodium concentrations of giant river prawn Macrobrachium rosenbergii adults were determined 15 days after they were transferred from freshwater to 0 (control), 7, 14 and 21‰ salinity. Haemolymph osmolality, chloride and sodium concentrations of prawns transferred to 7, 14 and 21‰ reached a constant level within 0.5, 2 and 3 days respectively. Both male and female adults exhibited strongly hyper‐osmotic and hyper‐ionic over the range of 0–14‰. The isosmotic point was 14.5 and 15.6‰, the iso‐ionic point of chloride was 14.5 and 14.7‰, and the iso‐ionic point of sodium was 16.5 and 15.7‰ for males and females respectively. The adults lost their hyper‐regulatory ability and became osmo‐conformic over the media of 14.5–21‰ and 15.6–21‰ and ion‐conformic with respect to sodium over the media of 16.5–21‰ and 15.7–21‰ for males and females respectively. The adults displayed hypo‐ionic to the media at 21‰ with respect to chloride.