Figure 1. The respiro-spirometer. (b) Diagram showing the assembled respiro-spirometer and measurement set-up. Water is continuously recirculated through the chamber by a peristaltic pump (1) drip feeding into a syringe barrel (2) where it is equilibrated with a regulated gas mixture of CO2-free air and pure N2 (3) passing through an airstone (4). Height of water in the syringe barrel is adjusted by a needle valve (5). A flow meter (6) and in-current O2 sensor (7a) record the water's flow rate and pO2 before it enters the respiration/ventilation chamber (8). A differential pressure transducer (PT) (9) records pressure within the chamber, while a 2.5 ml Hamilton syringe operated by a syringe pump (10) connected to the chamber through a separate port allows the PT to be calibrated in situ by periodically injecting/withdrawing known volumes of water at controlled rates. Water exits the chamber by flowing through an ex-current O2 sensor (7b) and, finally, a 15 gauge hypodermic needle that acts as a flow-resistive element (11). The thorax (12) and rear (13) harnesses are also shown.
Abstract
Dragonfly nymphs breathe water using tidal ventilation, a highly unusual strategy in water-breathing animals owing to the high viscosity, density and low oxygen (O2) concentration of water. This study examines how well these insects extract O2 from the surrounding water during progressive hypoxia. Nymphs were attached to a custom-designed respiro-spirometer to simultaneously measure tidal volume, ventilation frequency and metabolic rate. Oxygen extraction efficiencies (OEE) were calculated across four partial pressure of oxygen (pO2) treatments, from normoxia to severe hypoxia. While there was no significant change in tidal volume, ventilation frequency increased significantly from 9.4 ± 1.2 breaths per minute (BPM) at 21.3 kPa to 35.6 ± 2.9 BPM at 5.3 kPa. Metabolic rate increased significantly from 1.4 ± 0.3 µl O2 min−1 at 21.3 kPa to 2.1 ± 0.4 µl O2 min−1 at 16.0 kPa, but then returned to normoxic levels as O2 levels declined further. OEE of nymphs was 40.1 ± 6.1% at 21.3 kPa, and did not change significantly during hypoxia. Comparison to literature shows that nymphs maintain their OEE during hypoxia unlike other aquatic tidal-breathers and some unidirectional breathers. This result, and numerical models simulating experimental conditions, indicate that nymphs maintain these extraction efficiencies by increasing gill conductance and/or lowering internal pO2 to maintain a sufficient diffusion gradient across their respiratory surface.