Seeking refuge in subsurface microhabitats during aestivation aids avoidance of lethally high temperature and desiccation in the snail Helminthoglypta tudiculata (Binney, 1843) (Pulmonata: Helminthoglyptidae)

摘要

The threats to aestivating land snails posed by extremely high temperatures and prolonged desiccation are poorly understood. In this study, we used the southern California native snail Helminthoglypta tudiculata to investigate the role of microhabitat selection in avoiding lethally high temperatures during aestivation. We also examined water loss and metabolism during aestivation to understand the potential limits of water and carbon reserves for aestivating snails. In the field, searches were carried out both on the substrate surface (SS) and in subsurface refugia (SR) such as soft substrate, rodent holes or under rocks. All aestivating snails and the great majority of empty shells found in the field were in SR. In the laboratory, the lethal temperature for active snails in a 1-h exposure was 41.3 +/- 0.24 degrees C. Daily temperature highs measured over a 46-d period from September 2014 to November 2014 exceeded 41.3 degrees C at all 11 SS sites examined, but in only two of eight SR sites. SR aestivation sites provide protection from lethally high temperatures and water loss. Laboratory measurements of water loss rates for aestivating snails under standardized conditions indicated that, in the field, long-term water loss amounts to c. 244 mu g H2O h(-1). This loss rate is greater than that needed to ensure survival in the wild, suggesting that laboratory investigations of losses need to be regarded with caution. Measured CO2 fluxes in aestivating snails showed alternating periods of negligible flux punctuated by sharp increases, presumably coinciding with opening of the pneumostome, and decreased exponentially throughout aestivation. Water flux did not co-vary significantly with CO2 flux, indicating that the epiphragm maintains near-saturated humidity in the underlying air space. Water loss was proportionally greater than carbon (CO2) loss in long-term dormancy, suggesting that desiccation poses a greater threat than starvation to aestivating snails. Helminthoglypta tudiculata probably spends & 50% of the year in aestivation. Maintaining undisturbed ground surface with diverse refugia for aestivating land snails should be given appropriate priority in conservation efforts.