A novel technique for the precise measurement of CO2 production rate in small aquatic organisms as validated on aeshnid dragonfly nymphs.

The present study describes and validates a novel yet simple system for simultaneous in vivo measurements of rates of aquatic CO2 production (ṀCO2 ) and oxygen consumption (ṀO2 ), thus allowing the calculation of respiratory exchange ratios (RER). Diffusion of CO2 from the aquatic phase into a gas phase, across a hollow fibre membrane, enabled aquatic ṀCO2  measurements with a high-precision infrared gas CO2 analyser. ṀO2  was measured with a PO2  optode using a stop-flow approach. Injections of known amounts of CO2 into the apparatus yielded accurate and highly reproducible measurements of CO2 content (R2=0.997, P<0.001). The viability of in vivo measurements was demonstrated on aquatic dragonfly nymphs (Aeshnidae; wet mass 2.17 mg-1.46 g, n=15) and the apparatus produced precise ṀCO2  (R2=0.967, P<0.001) and ṀO2  (R2=0.957, P<0.001) measurements; average RER was 0.73±0.06. The described system is scalable, offering great potential for the study of a wide range of aquatic species, including fish.