ATMOSPHERIC WATER-ABSORPTION AND THE WATER-BUDGET OF TERRESTRIAL ISOPODS (CRUSTACEA, ISOPODA, ONISCIDEA)

Studies of terrestrial isopods (Crustacea, Isopoda, Oniscidea) have re vealed a capacity for active water vapor absorption (WVA) in the taxon omic sections Crinocheta and Diplocheta but not in Synocheta. Uptake t hresholds in Crinocheta are modest by comparison with other vapor abso rbers, but standardized uptake fluxes are among the highest recorded a nd are probably an adaptive requirement to counter the high transpirat ory losses. Comparative data for uptake fluxes, thresholds, and transp iratory 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 recov er 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 prediction s from ecotypic surveys. In the littoral Ligia oceanica (Diplocheta) h aemolymph hyperosmosis and periodic submergence provide additional mea ns of water balance regulation. It is proposed that WVA in Ligia provi des an essentially solute-free water source to counteract salt-loading in the splash-zone. The absence of WVA in synochetes, together with t heir cryptozoic habits, reflects an alternative terrestrial strategy t o those of other oniscideans.