Movement of European freshwater mussels under simulated dewatering conditions

Freshwater ecosystems are exposed to altered hydrological regimes due to climate change and poor water management, resulting in frequent and intense dewatering events. Freshwater mussels (Unionida) are particularly vulnerable to rapid water-level declines. Dewatering can lead to mortality through stranding and desiccation, yet inter- and intra-specific variation in responses is rarely assessed, in Europe, limiting conservation planning. We evaluated dewatering effects on three European freshwater mussel species (Unio delphinus, U. elongatulus, and U. pictorum), each represented by northern and southern populations. Mussels were exposed to slow, moderate, and fast dewatering rates and a control. Horizontal movement, burrowing, and strandedness (i.e. mussels completely emersed showing no response to external stimuli for a minimum of 1 h) were quantified. Horizontal movement did not differ from controls, although the northern population of U. pictorum showed decreased movement under fast dewatering. Burrowing varied among species and populations, with only the northern population of U. pictorum exhibiting significant differences in the fast dewatering treatment compared to the control group. Strandedness differed significantly among species, treatments, and populations, with U. elongatulus most affected overall. Our results reveal context-dependent behavioural responses to dewatering, highlighting the need to incorporate population-level variability into freshwater mussel conservation and flow management strategies.