Mechanisms of sustaining oxygen extraction efficiency in dragonfly nymphs during aquatic hypoxia

Despite breathing water using their tidally ventilated rectal gills, dragonfly nymphs show a surprising ability to maintain oxygen (O 2 ) extraction from the water during hypoxia. However, an increase in convective O 2 transfer is insufficient to sustain aerobic demands by itself, which suggests that diffusive mechanisms must also be involved. This study examines the contributions of changing the O 2 partial pressure gradient (PO 2 ) and/or O 2 conductance across the rectal gill in maintaining O 2 extraction efficiency (OEE) of dragonfly nymphs during hypoxia. Data were collected using the same custom-designed respiro-spirometer described in a previous study with the addition of an implanted O 2 sensor to measure hemolymph PO 2 . Results show that the implantation of the O 2 sensor does not affect the respiratory and ventilatory response of nymphs to hypoxia. Hemolymph PO 2 fell from 6.3 ± 1.6 kPa at normoxia to 2.5 ± 0.6 kPa at 16.0 kPa, which resulted in the PO 2 diffusion gradient remaining statistically constant at these two water PO 2 s (17.5 ± 1.7 and 15.4 ± 0.7 kPa during normoxia and 16.0 kPa respectively). Beyond 16.0 kPa, a progressive reduction in hemolymph PO 2 was unable to sustain the diffusion gradient. Mathematical modeling revealed that while the addition of hemolymph PO 2 in tandem with ventilation frequency was able to elevate OEE during 16.0 kPa to that of normoxia, both were still insufficient during severe hypoxia and required an increase in O 2 conductance. Estimating the change in whole-gill conductance showed that nymphs are indeed increasing their conductance as the water becomes hypoxic, demonstrating a reliance on both diffusion gradient and O 2 conductance to enhance diffusive O 2 transfer in conjunction with convective mechanisms to maintain O 2 extraction during hypoxia.