Which environmental factors influence the distribution patterns of an endangered freshwater mussel (Unio crassus)?

Freshwater mussels (Bivalvia: Unionida) are valuable components of river ecosystems. Among them, the thick-shelled river mussel (Unio crassus) is globally endangered, and its populations are decreasing in many European catchments. Over the last decade, it has become a focal point of environmental conservation programs. A comprehensive understanding of the relationship between the species and its environment is therefore required to optimize those programs. Yet, despite being increasingly studied, freshwater mussel ecological requirements remain complex to establish as they appear to be species- and region-related. We reviewed the potential environmental factors which have been showed to impact freshwater mussels, and therefore possibly U. crassus. We compiled datasets of environmental factors and occurrence of U. crassus across the study area (Wallonia, Belgium). Logistic regression models were built to identify the main driving factors. Results underscore the predominant significance of hydro-geomorphological variables in shaping distribution patterns. Specific stream power had a significantly negative impact on the distribution of U. crassus, while the probability of occurrence increased with catchment area, and with contrasted high- and low flows during the year. The high prediction accuracy of the hydro-geomorphological model supports the idea of a considerable influence of hydro-geomorphological habitat on the species. Other parameters are also relevant in predicting mussel distribution, like the percentage of land covered by meadows, elevated heavy metals concentrations and fish richness.