The freshwater bivalves Unionidae are in dramatic decline globally. Due to wide shell variability, only the genus can be determined by traditional morphological methods, with genetic analyses being required for species identification. From November 2018 to October 2020 four freshwater bodies in the Province of Bolzano/Bozen (Italy) (Small Lake and Large Lake of Monticolo/Montiggl, Lake Caldaro/ Kaltern, and its main emissary Fossa Grande di Caldaro/Großer Kalterer Graben) have been explored to collect unionid bivalves by hand and scuba diving. Unionid presence, bathymetric distribution, biometric characteristics, and attachment of invasive allochthonous zebra mussel (Dreissena polymorpha) were detected.
Molecular barcoding analyses (COI) on foot snips from 25 specimens (19 Anodonta and 6 Unio) confirmed the presence of four native species: Anodonta anatina, A. cygnea, A. exulcerata, and Unio elongatulus. Most Anodonta specimens from both Small (78 %) and Large Lake of Monticolo (60 %) were parasitized by Dreissena polymorpha in numbers ranging from 1 to 81 per individual. The biomass of D. polymorpha amounted up to 41 % of Anodonta’s weight. D. polymorpha was not found in Lake Caldaro/Kaltern and its emissary Fossa Grande di Caldaro/Großer Kalterer Graben.