The physiology of Venezillo arizonicus: water balance and the cuticular water barrier.

This study investigated the water balance physiology of Venezillo arizonicus, a land isopod endemic to the Southwest Desert Ecoregion of North America. Evaporative water losses were measured in two ways: gravimetric in dry air, and by perfusing animals with dry air in a respirometry chamber and monitoring downstream relative humidity (RH). The respective mean loss flux estimates were 0.140 and 0.177 μg h-1 cm-2 Pa-1, lower than other N. American Oniscidea described to date (≥0.491 μg h-1 cm-2 Pa-1). Humidity monitoring revealed intermittent sharp peaks in RH, attributed to the release of maxillary urine. Whole-animal flux increased as a function of temperature, with a critical temperature (Tc) between 38 and 42 °C identifiable by a clear Arrhenius break point. Hexane-rinsed whole animals yielded straight-chain, saturated hydrocarbons (C21-C33). The surface density of extracted alkanes (0.64 µg cm-2) was somewhat higher than previously determined values for mesic species, although modest compared to insects and arachnids. Dehydrated animals exposed to high RH (>88%) demonstrated active water vapor absorption (WVA) like other Crinocheta, with an extrapolated uptake threshold of approximately 85% RH (Aw = 0.85), the lowest value reported for Oniscidea. The maximum uptake flux increased linearly as a function of ambient RH. Mass-specific uptake in 100% RH was 49% d-1, similar to values determined previously for Armadillidium vulgare and Porcellio scaber. The low WVA threshold of V. arizonicus, and the species' low permeability, are consistent with its known range in the arid desert southwest and the desiccation stress of its typical aeolian sand and seasonal wash habitats.