Oxygen limitation and thermal tolerance in two terrestrial arthropod species

Recent studies of marine invertebrates and fish have suggested that lower and upper critical temperatures (CTmin and CTmax) are coupled by a common mechanism: oxygen and capacity limitation of thermal tolerance (OCLT). Using thermolimit respirometry, we tested the predictions of this theory for terrestrial arthropods by measuring maxima and minima for both critical temperatures and metabolic rate in two arthropods, the isopod Porcellio scaber and the beetle Tenebrio molitor, at 40%, 21%, 10% and 2.5% ambient O-2. Critical temperatures were identified as particular points on both activity and (V)over dot(CO2) traces in four ways. In the first two instances, we identified the inflection points in regressions of absolute difference sum (ADS) residuals calculated for activity (aADS) and (V)over dot(CO2) (VI), respectively. In the third, we visually identified the lowest point before the post-mortal peak in CO2 release (PMV). Finally, we pinpointed the sudden drop in (V)over dot(CO2) at death, where (V)over dot(CO2) fell outside the 95% confidence intervals of the 5 min period immediately preceding the drop-off (CI). Minimum and maximum metabolic rates were determined using CO2 traces, and the temperatures corresponding to these identified as T-MetMin and T-MetMax. For both species, ambient oxygen concentration did not influence CTmin, minimum metabolic rate, or T-MetMin. By contrast, severe hypoxia (2.5% O-2) caused a 6.9 degrees C decline in activity-based CTmax for T. molitor and a 10.6 degrees C decline for P. scaber, relative to normoxia (21% O-2). The magnitude of this decrease differed between methods used to estimated critical thermal limits, highlighting the need for a standard method to determine these endpoints during thermolimit respirometry. Maximum metabolic rate also declined with decreasing ambient oxygen in both species. The combination of increasing metabolic rate and oxygen limitation affected upper thermal limits in these arthropods only in severe hypoxia (2.5% O-2). In both species, CTmin and CTmax responded differently to oxygen limitation, suggesting that this is not a common mechanism coupling upper and lower limits in terrestrial arthropods.