A novel technique for the precise measurement of CO 2 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 ( Ṁ CO2 ) and oxygen consumption ( Ṁ O2 ), 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 Ṁ CO2  measurements with a high-precision infrared gas CO2 analyser. Ṁ O2  was measured with a P O2  optode using a stop-flow approach. Injections of known amounts of CO2 into the apparatus yielded accurate and highly reproducible measurements of CO2 content ( R 2 =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 Ṁ CO2  ( R 2 =0.967, P <0.001) and Ṁ O2  ( R 2 =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.