The effects of temperature, activity and convection on the plastron PO2 of the aquatic bug Aphelocheirus aestivalis (Hemiptera; Aphelocheiridae)

The aquatic bug Aphelocheirus aestivalis (Fabricius 1794) utilises a plastron, a thin bubble layer on the surface of its body to extract O-2 from the water. Millions of tiny hairs keep the bubble from collapsing, enabling the bug to remain submerged indefinitely. The development of fibre optic O-2-probes has allowed measurements of O-2 pressure (PO2) surrounding the plastron, and within the plastron although only for short periods. Here we developed methods to continuously measure plastron PO2, and investigate how it is affected by temperature (15, 20, 25 degrees C), activity, and water circulation. We also made measurements of water PO2, temperature and velocity in the field and swimming velocity at the treatment temperatures. Results show that plastron PO2 is inversely related to temperature, associated with differences in metabolic demand, and that small bouts of activity or changes in water convection result in rapid changes in plastron PO2. A model was developed to calculate the conditions under which Aphelocheirus would exist without becoming O-2-limited in relation to water temperature, PO2 and boundary layer thickness. This suggests that Aphelocheirus at one of two field sites may have a reduced metabolic scope even in well convected water in association with low PO2 and moderate temperature, and that in well convected, air-saturated water, bugs may have a reduced metabolic scope where water temperatures are between 20 and 25 degrees C. If exposed to 5 kPa PO2, Aphelocheirus cannot sustain resting metabolic rate even in well convected water and would die at temperatures above approximately 25 degrees C.