An improved method for the measurement of CO2 in marine animals,applied to three species of decapods

Previous methods for measuring CO₂ respired in sea water were inaccurate bacause of the high concentration of bound CO₂ (carbonates). The method developed here “refluxes” a fixed volume of air through a fixed volume of sea water while simultaneously trapping the respired CO₂. Tests were run long enough for the volume of respired CO₂ to approximately equal the CO₂ bound in the water, thus decreasing the former cause of inaccuracies. The method was tested with 3 species of crabs: Maia aquinado, Carcinus meanas, and Cancer pagurus. Tests lasted overnight (20 h). Six respirometers were used, 5 containing crabs and 1 control for the measurement of bound CO₂. The volume of respired CO₂ was given by the weight of trapped CO₂ (as BaCO₃) from the test units (the total of respired CO₂ and that in the test water) minus the weight of the control CO₂. In both fed and starved crabs, of all 3 species, CO₂ output followed the average trend of oxygen consumption, especially after feeding when both CO₂ and O₂ rose sharply. RQ's (respiratory quotients) could not be determined because O₂ was measured intermittently, but the relative ratio of CO₂ and O₂ was essentially unchanged throughout the experiments. In general, CO₂ was less variable than O₂ and may be a better indicator of average metabolic rate.