Abstract:Hemocyanin oxygen affinity and the fractionation of oxyhemocyanin and deoxyhemocyanin in the hemolymph of Penaeus monodon were studied for individual shrimps exposed to 0.001 (control), 0.07, 0.36, 0.72 and 1.44 mmol l−1 nitrite for 3 h up to 48 h in a salinity of 30 ppt at 25°C. The hemolymph P50 increased (indicating reduced affinity) and the hemolymph pO2 increased directly with ambient nitrite and exposure time. Hemolymph pH, oxyhemocyanin and hemolymph protein were inversely related to ambient nitrite and exposure time. P. monodon, following 6 h exposure to ambient nitrite at 0.36 mmol l−1, decreased significantly the fraction of oxyhemocyanin and increased that of deoxyhemocyanin. The increase of P50 and the increased fraction of deoxyhemocyanin suggests that hemocyanin oxygen binding decreases with a concomitant decrease of hemolymph pH, when P. monodon is subjected to nitrite.