Behaviour and physiology are linked in the responses of freshwater mussels to drought

1. Droughts have become prevalent in the south-eastern U.S.A. and are predicted to become more common in the future. Drought conditions have particularly strong negative effects on sessile aquatic organisms with limited dispersal ability. This study explored the linkages between physiological tolerance, behavioural response and survival of three species of freshwater mussels subjected to drought. 2. To assess physiological tolerance, we measured survival under desiccation at 25, 35 and 45 degrees C in the laboratory. To assess behavioural responses, we tracked horizontal and vertical movement of mussels in a drying stream reach. 3. Uniomerus tetralasmus showed the greatest desiccation tolerance, Lampsilis straminea was intermediate, and Pyganodon grandis had the lowest tolerance at all temperatures. 4. In the drying stream reach, U. tetralasmus showed little horizontal movement and quickly became stranded. The other two species tracked the receding water. It was not until the pool had been reduced to c. 10% of its original size that =50% of P. grandis and L. straminea became stranded. 5. Uniomerus tetralasmus and L. straminea burrowed in response to becoming stranded; however, both species burrowed only shallowly (34 cm), presumably because burrowing to greater depths did not convey increasing thermal refuge. Pyganodon grandis rarely burrowed. No P. grandis survived the 15-week drought, while 45% of L. straminea and 77% of U. tetralasmus survived. 6. Three strategies emerged for freshwater mussels to cope with drought conditions: tracking (intolerant), track then burrow (semi-tolerant) and burrowing (tolerant). Results suggest that drought poses the greatest threat to intolerant trackers, while tolerant burrowers are the most resistant to drought conditions.