Discontinuous gas exchange in the pseudoscorpion Garypus californicus is regulated by hypoxia, not hypercapnia

The discontinuous gas exchange cycle (DGC) of the pseudoscorpion Garypus californicus is characterized by periodic bursts of CO2 emission and by high rates of interburst CO2 emission. We investigated the mechanism that triggers the burst phase by manipulating ambient oxygen partial pressures (Po-2). The ventilatory trigger in most land animals is hypercapnia; in insects, for example, the burst phase is triggered when endotracheal Pco(2) reaches about 4 kPa. In insects with a DGC, hypoxia induces prolonged interburst phases because spiracular conductance is elevated to supply oxygen to the tissues, thus delaying the onset of the hypercapnia-triggered burst phase because CO2 accumulates more slowly. In G. californicus, hypoxia induced a decrease in interburst phase length, while hyperoxia increased its duration relative to normoxia. This is opposite to the condition in insects. In addition, CO2 emission fell during the interburst phase as ambient Po-2 rose, also opposite to the condition in insects. Thus, the burst phase is triggered in G. californicus (and presumably in other pseudoscorpions) not by hypercapnia but by hypoxia, a situation that is seldom encountered in terrestrial animals.