Abstract: Respiratory quotient (RQ) is one of the basic indices in physiology and energy metabolism of animals. When RQ is calculated, the amount of released CO₂ is typically used directly. But for calcifying marine organisms, calcification which can affect dissolved inorganic carbon (DIC) content in the water may cause methodological error to some extent, if it is ignored. In this paper, RQ and O/N of cultured oyster Crassostrea gigas and 3 marine fouling animal species (Mytilus edulis, Ciona intestinalis, Styela clava) were measured in the respiratory chamber to discuss the effect of calcification in RQ determination. The results demonstrated that calcification
rates of C. gigas and M. edulis were (56.37±14.85) and (17.95±7.21) μmol·g^-1·h^-1, respectively. (3.72±0.80) and (1.48±0.14) mg·L^-1 DIC in the water were correspondingly decreased, which occupied about (60.9±7.6)% and (39.9±5.7)% of respired CO2, respectively. RQ values of 4 animals were C. gigas 1.38±0.19, M. edulis 1.18±0.11, C. intestinalis 1.11±0.05 and S. clava 1.32±0.19, which agreed with the O/N values except C. intestinalis. Meanwhile, the uncorrected RQ values of C. gigas and M. edulis were 0.56±0.19 and 0.70±0.04, respectively, which were contrary to the O/N values. Therefore, it was obviously that calcification could result in a significant influence on the respiratory quotient by affecting water DIC concentration and should be accurately calculated in RQ measurement.