Life on the edge: survival and behavioural responses of freshwater gill-breathing snails to declining water level and substratum drying

1. Reductions in water level and temporary exposure of substratum to air are two of the most important factors structuring communities in temperate near-shore zones. Snails are suitable model organisms to study responses of aquatic communities to water-level fluctuations, as they often inhabit near-shore zones and are relatively mobile. 2. In a laboratory study, we investigated survival and behavioural responses (horizontal migration and burrowing) of three freshwater gill-breathing snails: Viviparus viviparus (L.) (large and small individuals), Bithynia tentaculata (L.) and Potamopyrgus antipodarum (Gray) to substratum drying. We studied the effect of distance to a suitable habitat (24 and 48 cm) and rate of dewatering (gradual: eight days or sudden) on horizontal migrations of snails as well as their burrowing depth. 3.Large V.viviparus individuals were best adapted to dewatering: they were resistant to drying (surviving more than 3weeks of air exposure) and migrated in response to decreasing water leveland/or burrowed into the substratum. Their horizontal movement was negatively related to therate of dewatering and distance needed to get to a submerged area. However, they were able to migrate to some extent even after a sudden dewatering and to reach a distant submerged refuge.Small V.viviparus exhibited similar behavioural responses as large individuals, but were lessresistant to drying, surviving approximately one week of air exposure. Neither B.tentaculata nor P.antipodarum exhibited behavioural adaptations to desiccation, although resistanceto drying differed markedly from approximately 3weeks for B.tentaculata to 3-4days for P.antipodarum. 4.Resistance to drying and behavioural adaptations are two factors enabling benthic organisms to survive in harsh near-shore environments. Our study revealed marked intra- and interspecific differences in response to dewatering, depending on body size, distance to a refuge and rate of dewatering. Appropriate physiological and behavioural responses may increase fitness of organisms under changing environmental conditions and contribute to their survival.