Atmospheric Water Absorption and the Water Budget of Terrestrial Isopods (Crustacea, Isopoda, Oniscidea)

Studies of terrestrial isopods (Crustacea, Isopoda, Oniscidea) have revealed a capacity for active water vapor absorption (WVA) in the taxonomic sections Crinocheta and Diplocheta but not in Synocheta. Uptake thresholds in Crinocheta are modest by comparison with other vapor absorbers, but standardized uptake fluxes are among the highest recorded and are probably an adaptive requirement to counter the high transpiratory losses. Comparative data for uptake fluxes, thresholds, and transpiratory losses allows the compilation of water budgets in hypothetical temperature and humidity regimes. Given a 12-h light-dark cycle, with saturated ambient activities for diurnal WVA, all species could recover water losses incurred during nocturnal foraging in an ambient water activity of 0.75, and xeric species could forage in activities below 0.30. Xeric trends based on these models agree closely with predictions from ecotypic surveys. In the littoral Ligia oceanica (Diplocheta) haemolymph hyperosmosis and periodic submergence provide additional means of water balance regulation. It is proposed that WVA in Ligia provides an essentially solute-free water source to counteract salt-loading in the splash-zone. The absence of WVA in synochetes, together with their cryptozoic habits, reflects an alternative terrestrial strategy to those of other oniscideans.