The discontinuous gas exchange cycle of the pseudoscorpion Garypus californicus, mean mass 5.9 …

• Abstract
• Introduction
• Materials and Methods
• Results
• Discussion
• References
• Figures

We analyzed 771 DGCs by an approximately equal number (10-11) of haphazardly sampled animals at 15, 25 and 35 °C, with a mean mass of 5.90 1.28 SD mg. Larger numbers of DGCs were analyzed at higher temperatures because they occurred at a faster rate. We analyzed 137 DGCs from 11 animals at 15 °C, 181 DGCs from 10 animals at 25 °C, and 453 DGCs from 11 animals at 35 °C. The data were analyzed by individual within each temperature treatment. Because the pseudoscorpion colony had approximately 19 inhabitants, and because of the random (or more precisely, haphazard) selection technique, i.e. selecting a new individual at random and then replacing the previously measured individual, some pseudoscorpions will have been measured more than once in different, but never consecutive, runs. This is an inevitable consequence of the restricted pool size of our experimental organisms. We have no reason to believe that our sampling technique introduced any bias, by virtue of its inherently random nature.

Typical CO₂ emission data by a 6.02 mg G. californicus at 15 °C are shown in Fig. 1. The DGC in our pseudoscorpions was generally characterized by incomplete spiracular closure during the constricted spiracle or "C" phase, which therefore is something of a misnomer in this group. We retain the term because spiracular closure was, on occasion in some individuals, close to or indistinguishable from zero at low temperatures. In addition, in no arthropod are the spiracles totally constricted. The reader may wish to think of the C phase as used here as an "interburst phase." No evidence was found for an F (fluttering spiracle) phase. For purposes of analysis we divided the DGC of G. californicus into the C and O (Open-spiracle) phases; again, we remind the reader that these terms should be understood as shorthand references to the interburst and burst phases of the DGC, respectively, rather than as references to conventional C and O phases.

Overall metabolic flux rates

At 25 °C, the mean VCO₂ of our animals was 1.126 0.567 SD µl hr^-1, or 0.201 0.101 SD ml g^-1 hr^-1, N = 181 DGCs by 10 animals, mean mass 5.65 1.31 mg. Assuming aerobic catabolism of palmitate (respiratory quotient 0.72), this corresponds to a metabolic rate of 8.44 4.25 SD µW, which does not differ significantly from the value expected for a consensus arthropod of that mean body mass and temperature (Lighton et al., 2001); 11.5 µW (t = 0.68; P = 0.4).

As might be expected because of the small range of body masses, there was no correlation between body mass and metabolic rate (ANCOVA of log body mass vs. log metabolic rate across the three temperature treatments, F[1, 28] = 1.43, P[no correlation] > 0.2).

By regressing the logarithm of metabolic rate against temperature, the slope of that relationship (0.0336) was found to correspond to the temperature sensitivity of metabolism. Expressed as a Q₁₀, it is 10^0.336 or 2.16. Using this model, temperature explains 72% of metabolic rate variance (F[1, 30] = 76.9; P

DGC Frequency

The DGC frequency increased exponentially with temperature, with a slope of 0.0261 for log-transformed frequency vs. temperature, corresponding to a Q₁₀ of 10^0.261 or 1.82; temperature explained 57% of DGC frequency (F[1, 30] = 39.5; P 2.66 SD mHz, corresponding to a mean cycle length of 193 seconds or slightly over three minutes.

C Phase

C phase duration increased with DGC duration (Fig. 2), with a dimensionless shared slope of 0.64 across temperatures (r² = 0.71; F[1, 30] = 75.4; P 61 seconds, which does not differ significantly from zero. C phase length decreased significantly (P

C phase VCO₂ increased steeply and significantly with temperature, with a Q₁₀ of 2.75 (r² = 0.70, F[1, 30] = 68.5, P ₂ with temperature, its contribution to total CO₂ release increased with temperature (Q₁₀ = 2.75/2.16 = 1.27) until at 35 °C it approached 45% of total CO₂ emission.

O Phase

O phase duration increased with DGC duration, with a dimensionless shared slope of 0.36 across temperatures (r² = 0.45; F[1, 30] = 24.5; P 61 seconds, which does not differ significantly from zero. O phase duration was strongly affected by temperature, decreasing from 201 24 SE seconds at 15 °C to 106 17 SE seconds at 25 °C and 71 8 SE seconds at 35 °C (Fig 3). O phase volume rose modestly between 15 and 25 °C, from 3.06 0.41 SE µl mg^-1 to 3.63 0.52 SE µl mg^-1. It then declined steeply at 35 °C to 2.14 0.24 SE µl mg^-1. Thus the effect of temperature on O phase volume, though marginally significant by ANOVA (F[2, 29] = 3.48; P ₂, in contrast, increased exponentially with temperature (F[1, 30] = 45.3; P ₁₀ of O phase VCO₂ was 1.97.