TURNER, Robbin J; LIGHTON, John R; Sable Systems International; University of Nevada, Las Vegas: Bigger is better: The interactions of water relations, body size and metabolism in the terrestrial isopod, Armadillidium vulgare.

The broadly distributed woodlouse, Armadillidium vulgare is a common representative of mesic to mesic-xeric habitats. These crustaceans� adaptations for water balance � loss, gain and retention � are a physiological priority. This isopod�s permeability is significantly lower than that of other mesic isopods. Measurement of A. vulgares� water vapor uptake through water vapor absorption (WVA) immediately following desiccation to varying absolute losses and at different loss rates allowed for the allometric modeling of its survival capacity and maximum exposure duration as a function of temperature and relative humidity. Lower relative humidities (RH) cause faster desiccation but trigger a significant decrease in whole-animal water loss after controlling for saturation deficit. The isopods� metabolic rates and activity levels increase dramatically at humidities below 65%. Size (mass) strongly affects survival duration. Although water loss scales with mass, larger isopods survive disproportionately longer at low RH�s; growth to some optimal size >80 mg conveys a clear selective advantage in survival duration. We evaluate the potential for pressure cycling and explore the evolutionary and ecological correlates of our findings in light of the paradoxical distribution of terrestrial isopods.