Desiccation resistance in Sphaerium occidentale and Musculium securis (Bivalvia: Sphaeriidae) from a temporary pond

The fingernail clams Sphaerium occidentale and Musculium securis often inhabit the same temporary pond. Sphaerium occidentale occurs only in temporary aquatic habitats and M. securis in both temporary and permanent aquatic habitats. Both burrow into the sediment and aestivate when the habitat dries in summer. While aestivating, S. occidentale is more resistant to drying than M. securis, although both species require relative humidities near 100% to survive prolonged atmospheric exposure. A soil moisture level of at least 15% by weight is required by both species to resist desiccation during aestivation in the laboratory. In the dry pond, microhabitats characterised by moist, organic soils support the highest survival rates for both species. To survive aestivation, resistance to desiccation is developed by all length classes of S. occidentale. Only newborn M. securis are able to aestivate.