Effects of temperature and oxygen availability on water loss and carbon dioxide release in two sympatric saproxylic invertebrates

Water loss and VCO2 relative to temperature and oxygen tension was investigated in a log-dwelling onychophoran (Euperipatoides rowelli) and a sympatric, un-described millipede species using flow-through respirometry. Onychophorans possess a tracheal system featuring permanently open spiracles. Total body water loss was consistently very high in E. rowelli and there was a positive correlation with increasing temperature. CO2 output was continuous, increasing with higher temperatures and decreasing under lower oxygen concentrations. The millipede which has occludible spiracles also showed continuous gas exchange; however water loss was up to an order of magnitude lower than E. rowelli. An ability to survive under hypoxia is apparent for both species and corresponds with reports of hypoxic conditions within rotting logs. The rotting log habitat common to both taxa is characterized by high relative humidity and typically cool temperatures that approach 0 °C at night in winter. Consequently, dispersal through the higher temperatures and lower humidity of the exposed and dry understorey between suitable habitat may be hazardous for E. rowelli due to high desiccation susceptibility.