Physiological Adjustments during Aestivation by the Australian Land Snail Rhagada tescorum (Mollusca : Pulmonata : Camaenidae)

Abstract

Specimens of the camaenid snail Rhagada tescorum were collected from Barrow Island, Western Australia, where they typically aestivate underground during the long dry season and emerge after heavy rainfall. Aestivation is associated with a profound decrease in metabolic rate (from 50 to 5 µL g^-1 h^-1 ) and evaporative water loss (from 11 to 0.3 mg g^-1 h^-1). Even after approximately 10–12 months of aestivation in the laboratory, aestivating snails had only a slightly lower body-water content (82.1% of shell-free mass) than rehydrated individuals (84.5%), but a significantly higher haemolymph sodium concentration (94 cf. 55 mM) and osmotic concentration (233 cf. 134 mOsm); the difference in haemolymph potassium concentration (3.7 cf. 2.6 mM) was not significant. The haemolymph pO₂ was lower for aestivating snails (6.25 kPa) than awake snails (10.8 kPa) but there was no hypercapnia (pCO₂ = 1.4 cf. 1.3 kPa) or acidosis (pH = 7.64 cf. 7.62) in aestivating snails. These physiological adjustments of Rhagada tescorum during aestivation are similar to those of other arid-zone land snails, except for the absence of hypercapnia and acidosis, and indicate that this snail is well adapted to withstand the metabolic and hygric demands of extended periods of drought.