Abstract:The diurnal rhythm of respiration, excretion and calcification of Crassostrea gigas at three sizes(shell height：Small: 2.5 cm; Middle: 5.5 cm; Big: 6.8 cm) were studied in laboratory control (at 10℃and 20℃) and an experimental farming area(at 18℃). A rhythm was found for the metabolism of oysters. Oxygen consumption (OR) showed a diurnal rhythm, with higher OR at night at 10℃ in the experimental containers， and a reversed rhythm was found at 18℃ in the sea area. The difference in OR between day and night was 0.07~0.08 mg/ind•h under either situations. Meanwhile, OR at 20℃ was stable. The NH3 excretion rate showed the same trend at day and night, which was different with the OR rate. It was considered that excretion of C. gigas may be affected by tidal rhythm. Calcification did not show an obvious rhythm, but it was significantly different between time periods (P＜0.05). Reasons for the difference in metabolism rhythm was discussed in this paper. It is suggested that calculating the level of metabolism by physiological indices gathered in a short interval should be avoided,and replicates at different time periods are needed.
