Habitat functions of freshwater mussel shells for riverine macroinvertebrates

Freshwater mussels are keystone species in aquatic ecosystems and the presence of living mussels can enhance ambient macroinvertebrate biodiversity. However, due to a lack of empirical data, the functional role of dead freshwater mussel shells as habitat for other species remains unclear. Drawing primarily from research in marine ecosystems, we hypothesized that mussel shells enrich riverbed structure by providing diverse microhabitats, especially for macroinvertebrates. We conducted a field experiment in which shells of native freshwater mussel species (Margaritifera margaritifera, Unio crassus, Anodonta spp.) were exposed using wire cages in three streams with occurrence of mussels for two months in spring and autumn. Colonization by macroinvertebrates was compared to controls without shells. An effect of mussel shells was observed in each stream. Although overall the differences between streams and seasons exceeded the effect of exposing mussel shells on the macroinvertebrates community level, there was a positive trend for a higher abundance of predators and filter feeders in the cages with mussel shells. Notably, there was a clear positive effect of mussel shells on specific species, such as two crayfish species Astacus astacus and Pacifastacus leniusculus. The findings of this study indicate that dead shells provide important microhabitat functions if a threshold volume of shells (>0.1 m² shell surface with a surface ratio of 1.6 shell surface/riverbed) is exceeded and shells should therefore be retained in aquatic ecosystems. Additionally, mussel shells can be utilized to promote target species for conservation such as crayfish.