We investigated the hemolymph oxygen and acid–base status of Akoya pearl oysters, Pinctada fucata martensii, exposed to hypoxic seawater to elucidate the acid–base balance. Akoya pearl oysters cannulated to the anterior aorta for hemolymph collection from the submerged animals showed oxygen and acid–base disturbance of the hemolymph during environmental hypoxia for 24 h (O_2 partial pressure in seawater, Pwo_2 8 torr). The hemolymph O_2 partial pressure (Po_2) decreased from 72.2 torr to 13.6 torr, pH decreased from 7.581 to 7.129, and CO_2 partial pressure (Pco_2) increased from 0.86 torr to 3.31 torr during hypoxia. The hemolymph total CO_2 concentration (Tco_2) and bicarbonate ion concentration ([HCO_3^–]) were 1.93–1.95 mM/L and 1.80–1.91 mM/L, respectively, and there was no statistically significant change between pre-hypoxia and hypoxia for 24 h. When normoxic seawater was resumed after the hypoxia, the hemolymph Po_2, pH, and Pco_2 returned to their initial levels for about 3 h, and hemolymph Tco_2 and [HCO_3^–] gradually increased. These results showed that Akoya pearl oysters undergo hypoxemia and respiratory acidosis in the hypoxic environments for 24 h (Pwo_2 8 torr). In post-hypoxia, most of the disturbances disappeared within 3–24 h, and the increase in hemolymph [HCO_3^–] which was a secondary change compensated for respiratory disturbance.

We investigated the hemolymph oxygen and acid–base status of akoya pearl oysters, Pinctada fucata martensii, exposed to air for a short time (4 h) to elucidate the acid–base balance and CO_2 dynamics. The hemolymph O_2 partial pressure (Po_2) in air-exposed akoya pearl oysters decreased from 88.7 torr (mean value) to 29.4 torr at 1 h, and the low Po_2 continued for the next 3 h during air exposure. The hemolymph pH decreased from 7.586 to 7.082 during air exposure for 1 h and reached 6.851 at 4 h. The hemolymph CO_2 partial pressure increased from 0.9 torr to 4.4 torr at 1 h and reached 7.3 torr after 4 h of air exposure. The hemolymph bicarbonate concentration and calcium ion concentration at 0 h (control) were 1.9 mM/L and 9.0 mM/L, respectively, and these properties did not significantly change during air exposure. From these results, it was determined that the akoya pearl oysters had hypoxemia caused by hypoventilation at an early phase of the short-term air exposure. The akoya pearl oysters inhibited the discharge of CO_2 by hypoventilation, and respiratory acidosis was caused due to the excessive accumulation of CO_2. Bicarbonate was not mobilized from the shell valve into the hemolymph during the short-term air exposure.