Abstract
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.This study analyzed 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. Pseudanodonta complanata was the dominant unionid species in Lake Siecino reaching densities of up to 26 ind/m2, 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 Unio tumidus were found at depths < 1 m, indicating different habitat preference.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; 4.22 and 0.86 ± 1.87; 8.76 in P. complanata and U. tumidus, respectively. Pseudanodonta 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.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.
Highlights
Due to their important ecosystem functions (e.g., Lummer, Auerswald, & Geist, 2016; Vaughn, 2018) and their ongoing declines (Lopes-Lima et al, 2017), conservation, and restoration of freshwater mussel populations are high on the agenda in Europe and elsewhere (e.g., Geist, 2011, 2015; Geist & Hawkins, 2016)
P. complanata can occur to a depth of 11 m (Van Damme, 2011), 76% of individuals in our study occurred above 5.5 m, that is, above the current thermocline
Our study shows that fouling intensity was negatively correlated with unionid density: the mass of D. polymorpha per individual P. complanata was highest at the lowest density of this unionid, with the pattern following a threshold-distribution rather than a linear relationship
Summary
Due to their important ecosystem functions (e.g., Lummer, Auerswald, & Geist, 2016; Vaughn, 2018) and their ongoing declines (Lopes-Lima et al, 2017), conservation, and restoration of freshwater mussel populations are high on the agenda in Europe and elsewhere (e.g., Geist, 2011, 2015; Geist & Hawkins, 2016). Pseudanodonta complanata inhabits lotic environments, including lowland and mountainous rivers, large drains, and canals (Bonk, 2019; Killeen, Aldridge, & Oliver, 2004) and has been reported from some lakes (Van Damme, 2011). Its geographical range extends across most of Europe, but its populations are usually small, scattered, and isolated. Most often, it co-occurs with other unionids but is the least abundant species, usually contributing to less than 5% of specimens in unionid assemblages (Piechocki & WawrzyniakWydrowska, 2016; Zettler, 1998, 1999). Based on a small number of population assessments, strong population declines and fragmentation of P. complanata distribution were recorded (Skidmore, Leach, Hoffman, Amos, & Aldridge, 2010; Van Damme, 2011; Zając, 2009). Pseudanodonta complanata is currently listed as vulnerable in the IUCN Red List, is legally protected in Germany and Poland, and is a species of conservation priority on the UK Biodiversity Action Plan
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