Sexual dimorphism in desiccation responses of the sand scorpion Smeringurus mesaensis (Vaejovidae)

The osmoregulatory and respiratory responses of male and female Smeringurus mesaensis (Vaejovidae) to prolonged desiccation were measured. No significant effect of sex on mass-loss rates (MLRs) was found. Still, females maintained their haemolymph osmolality when desiccated to 10% mass loss, whereas that of males increased significantly after loss of as little as 5% of initial mass. Females had a 3-fold larger hepatopancreas, significantly higher hepatopancreas water content and higher metabolic rates when adjusted to hepatopancreas-free dry mass. Thus, females not only store more water in the hepatopancreas but also mobilise it to the haemolymph at a higher rate during desiccation, thus maintaining haemolymph osmolality. Gas exchange rates of both males and females decrease as desiccation progresses. An initial respiratory exchange ratio (RER) of ∼0.9 is followed by a significant increase at mass loss levels of 7.5% and higher. RER values greater than 1.0 may result from partial shift to anaerobic catabolism, which allows closure of the book lung spiracles for longer duration, thus minimising respiratory water loss. The effects of gas exchange rates on rates of water mobilisation between body compartments and water loss to the environment suggest a trade-off between maintaining osmotic stability and conserving body water stores under stressful conditions.