Effects of dissolved oxygen on hemolymph parameters of freshwater giant prawn, Macrobrachium rosenbergii (de Man)

Profiles of changes in physiological parameters of freshwater giant prawns, Macrobrachium rosenbergii, exposed to various dissolved oxygen (DO) levels of 7.75, 4.75, 2.75, and 1.75 mg l^?1 are reported. The parameters involved in osmoregulation and oxygen transport were monitored for a 6-day period. Notable depressions in hemolymph osmolality, Na⁺, K⁺, and Cl^? contents were observed within 24 h after exposure to hypoxia at 2.75 and 1.75 mg O₂ l^?1, and thereafter remained at rather steady levels, which were significantly lower than those under normoxic conditions (4.75–7.75 mg O₂ l^?1). The extent of depression of osmotic-related constituents, hemolymph osmolality and Cl^? in particular, increased with decreased DO.

Oxyhemocyanin constituted 65.46–65.84% of total hemolymph proteins under the various DO levels examined; both hemolymph oxyhemocyanin and proteins showed notable elevations 24 h after exposure to hypoxic conditions, and reached the highest and constant level by 48 h after exposure. The compensatory responses of prawns to reduced O₂ were manifested by increased O₂ uptake through augmentation of hemocyanin, which results in enhancement of oxygen binding capacity of the hemolymph. In the same period, a significant surge of the respiratory products, PCO₂ and HCO₃^?, was also demonstrated 6 h after exposure to hypoxic conditions which resulted in hemolymph alkalosis. These processes likely resulted in an increase in water influx and consequent declines in hemolymph osmolality and ion composition. Furthermore, hyperventilation and respiratory alkalosis, indicated by increased oxyhemocyanin and pH, respectively, were found to be predominant responses of M. rosenbergii to hypoxic stress.