Total Ammonia Concentration and P^H of Haemolymph, Pleon Fluid and Maxillary Urine in Porcellio Scaber Lattreille (Isopoda, Oniscidea): Relationships to Ambient Humidity and Water Vapour Uptake Available to Purchase

Ammonia levels and pH were studied in body fluids of Porcellio scaber with special reference to ambient humidity and the timing of pleon fluid secretion and pleopodal ventilation. In high humidity (approximately 96.7%), concentrations of total ammonia (NH₃⁺NH₄⁺) in haemolymph display pronounced episodic changes, ranging from less than 1mmol l⁻¹ to more than 50mmol l⁻¹. Elevated concentrations (>5mmol l⁻¹) occur in discrete bouts lasting from 40min to more than 2h. Concomitant changes in total ammonia levels are seen in pleon fluid and maxillary urine. Overall, mean ammonia concentrations in pleon fluid, but not maxillary urine, are significantly higher than in the haemolymph, which may indicate active secretion of NH₄+ in the pleon. Elevated ammonia concentrations are not observed in reduced ambient humidities (85% and 30%). Evidence supports a physiological coupling between pleon NH₃ volatilization and active water vapour absorption. Pleon fluid and haemolymph pH values are similar, discounting a role of alkalization in NH₃ volatilization. Published rates of ammonia excretion are compatible with intermittent volatilization from pleon fluid and with measured NH₃ levels during elevations in ammonia concentration.