1. Freshwater mussels are in decline worldwide, with the depressed river mussel Pseudanodonta complanata being one of the rarest and most endangered species in Europe. Invasive mussels are suspected to be an important factor of decline, but there is little information on their interaction with native species.
2. This study analysed densities, depth distribution and individual sizes and weights in one of the largest known populations of P. complanata in Europe in relation to the co-occurring invasive zebra mussel Dreissena polymorpha and other mussel species, using a systematic transect analysis.
3. P. complanata was the dominant unionid species in Lake Siecino reaching densities of up to 26 ind. m^-2, with half of the specimens found at a water depth of 2.0 – 4.0 m. Densities were highest on sandy substrates in areas of underwater currents. In contrast, 67% of native U. tumidus were found at depths < 1 m, indicating different habitat preference.
4.  In the study area, 91% of P. complanata, 92% of U. tumidus and all Anodonta individuals were fouled by D. polymorpha. The dreissenid:unionid mass-ratio (mean ± SD; maximum) was 0.43 ± 0.56; 22 and 0.86 ± 1.87; 8.76 in P. complanata and U. tumidus, respectively. P. complanata fouled with D. polymorpha were impaired in their anchoring capability and had shell deformations potentially affecting shell closing and filtration activity. Fouling intensity was negatively correlated with unionid density, potentially leading to accelerated population declines.
5. The observed adverse effects of invasive zebra mussels on the depressed river mussel and the difficulties in eradicating established populations of invasive mussels suggest that D. polymorpha should be considered a serious threat to P. complanata. Therefore, the further spread of zebra mussels into habitats with native unionids needs to be avoided by all means.

SI Table 1. Data on individual mussels in study sites S1 - S5. Aa, Anodonta anatina; Ac, Anodonta cygnea; Pc, Pseudanodonta complanata; Ut, Unio tumidus; Depth, water depth at which the mussel was collected; Mass, total wet mass (body and shell); Mass_Dp, mass of D. polymorpha attached to the unionid