ABSTRACT
A capacity for active water vapour absorption (WVA), a phenomenon hitherto described only for acarines and insects, has recently been demonstrated in terrestrial isopods (Isopoda, Oniscidea). Rapid vapour uptake in humid diurnal retreats allows many species to replenish substantial water losses sustained during nocturnal foraging. In theory, this should permit exploitation of relatively low foraging humidities. The uptake rates in saturated air, when expressed as g H2O h ' or as g H2O h ' per unit absorbing surface area, are the highest documented for known vapour absorbers. Absorption takes place in the pleon, and is accomplished by two complimentary mechanisms. Secretion of strongly hyperosmotic NaCl (up to 8.2 Osm kg"1) into the pleoventral cavity generates a colligative lowering of vapour pressure, permitting vapour absorption above threshold humidities of 86.6% to 92.7% RH. Uptake rates can be increased and thresholds lowered by an additional facultative process involving compression of air beneath the imbricate pleopodal exopods. This serves to elevate the humidity of the enclosed volume of air, and represents the first demonstration of pressure cycling as a vapour absorption mechanism. Nitrogenous excretion, like WVA, plays an important role in the water economy of terrestrial isopods. Unlike the majority of insects and arachnids, they have retained ammonotelic excretion, characteristic of their marine ancestors. They thus constitute an exception to the broadly accepted dogma that ammonotely is restricted to hygric or aquatic groups. Although elimination of ammonia in gaseous form represents a means of minimizing concomitant water loss, the need to retain permeable regions of the integument for NH3 diffusion inevitably compromises water conservation. The site and mechanism of ammonia volatilization have previously been the subject of controversy. Recent work has established a clear association between periods of ammonia volatilization and the occurrence of highly elevated concentrations of ammonia in both the haemolymph and the external pleon fluid which bathes the respiratory endopods. Volatilization appears to be driven by the large concentration gradients between pleon fluid NH3 and the atmosphere; the pNH3 is not augmented by alkalinization of pleon fluid. Bouts of ammonia volatilization also show a clear temporal association with metachronal beating of the pleopods. The substantial bursts of haemolymph ammonia coincident with volatilization require the rapid mobilization of ammonia
from a precursor. Current studies indicate a primary role for arginine, glycine, glutamine and glutamate as precursors. Several lines of evidence support a possible physiological coupling between ammonia volatilization and WVA. It is probable that vapour uptake is primarily diurnal, as has been clearly documented for ammonia volatilization. Volatilization has not been observed in humidities below the thresholds for WVA and, like WVA, is invariably associated with pleopodal ventilation. Coupling of the two processes, whether obligate or facultative, would provide a unique example of ammonotely actually being associated with a net gain of water. Possible evolutionary scenarios favouring retention of ammonotely along with the origins of water vapour absorption in isopods are discussed.
