Disentangling the different role of parasites and invasive species in the decline of native mussels

Parasites have important negative consequences for freshwater mussels, but in the past they have been regarded as non-lethal and without any drastic impact upon mussels populations. Nonetheless, increased parasite densities impair mussel physiological conditions, which causes an increased mortality rate in mussels exposed to other stressors. A recently reported stressor is the spread of non-indigenous species (NIS), that can introduce new parasites and/or alter the dynamics of endemic parasites. To explore the interaction between biological invasions and growing parasites presence we profited of the sharp decline of the dominant native mussel (Unio elongatulus) in Lake Maggiore (Northern Italy), after the arrival of Corbicula fluminea and Sinanodonta woodiana. This study addressed whether: 1) the native species decline was determined by NIS co-introduction of own parasites; 2) introduced species triggered endemic parasite emergence; 3) habitat characteristics affect the interacting impacts of NIS and parasites. To answer these questions we evaluated: 1) the occurrence of parasites (alien/endemic) and the frequency of shared parasites in invasive and native bivalve species; 2) the prevalence and intensity of parasitic diseases in native species: a. before and after NIS arrival (task 2); b. in different habitat types and at different depths (task 3)