Spatial aggregation of native with non-native freshwater bivalves and activity depletion under summer heat waves: 'dangerous liaisons' in a climate change context.
Freshwater mussels are considered sedentary organisms. However, their foot provides the organ of locomotion, which enables an active behaviour. Their behaviour includes righting, horizontal movements and burrowing into the substrate. The objective of this study was to analyse the influence of non-native (Corbicula fluminea) competitors on the pedal activity of native (Unio delphinus) freshwater mussels in a climate change context. The pedal activity was scored and compared among different species compositions, population density and exposure to heat wave progression. The results showed a significant effect of species composition and heat wave progression. In addition, a significant effect of species composition × population density was found. Mussels at low density conditions exposed to dense aggregations of C. fluminea showed lower pedal activity than individuals in intraspecific treatments, or individuals at high density conditions exposed to low densities of C. fluminea. Maximum pedal activity was recorded at the thermal peak. After that, pedal activity was reduced when high temperature was maintained over time. Altogether, present results suggest that lower pedal activity at high C. fluminea densities may expose mussels to more intense interspecific competition. In addition, heat waves may compromise mussels' ability to locate refuge areas in front of receding waters.