![]() Water temperature decreased from above 20☌ in the epilimnion to below 10☌ in the hypolimnion, with a distinct thermocline between 5 and 7 m. The values of physicochemical water parameters were similar at both measurement sites. The measurements were taken once in the area of the highest depth of the lake (N 53.615994, E 16.024083) on 06 June 2018 at 15:40, and once in the area approximately 100 m offshore from the site where most P. complanata were found (N 53.587288, E 16.019255) on 07 June 2018 at 10:00. We assessed the physicochemical parameters of the water with a WTW Multi 3430 equipped with a IDS depth probe with oxygen, pH and conductivity (Xylem Analytics Germany Sales GmbH & Co. Specifically, we hypothesized that (a) P. complanata would have density maxima at places with lotic conditions in the lake, differing from the distribution of other species such as Unio tumidus, (b) specimens of P. complanata would be less heavily fouled by D. polymorpha than other native mussel species due to their burrowing behavior, but (c) fouling of P. complanata with D. polymorpha would result in adverse effects on P. complanata as evident from shell deformities and difficulties of specimens to anchor and burrow into the substrate.Ģ.2. The core objective of this study was to characterize the depth‐ and size‐distribution of endangered P. complanata from Lake Siecino in relation to the loading with invasive D. polymorpha. Adverse effects of the globally invasive D. polymorpha on the physiological condition of native mussels (Sousa, Pilotto, & Aldridge, 2011) as well as on population declines (Strayer & Malcom, 2018) are well established, but to the best of our knowledge, the effects of D. polymorpha on P. complanata have not yet been analyzed. For instance, the Chinese pond mussel Sinanodonta woodiana was found to potentially outcompete European Anodonta species by a broader host fish use, faster metamorphosis and higher recruitment success as evident from both laboratory (Huber & Geist, 2019) and field studies (Urbańska, Kirschenstein, Obolewski, & Ożgo, 2019). Generally, invasive mussel species have been proposed to infer with native mussels (e.g., Strayer, 1999), but there are only few examples where this interaction has been systematically studied. Dreissena polymorpha attaches with byssal threads to all kinds of hard substrates, including mussel shells (Mackie, 1991). obs.) the initial time of invasion is unknown. The invasive zebra mussel Dreissena polymorpha was introduced into this lake, and its abundant occurrence has been observed since the early 1980s (RT, pers. This provided an excellent opportunity to study population characteristics and habitat use of this little known species. ![]() In 2015, we discovered one of the largest known populations of this species in Lake Siecino in northern Poland (Ożgo, 2016), a postglacial lake with a size of 743 ha. 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. Further declines are predicted under climate change scenarios (Gallardo & Aldridge, 2013). 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). 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 & Wawrzyniak‐Wydrowska, 2016 Zettler, 1998, 1999). Its geographical range extends across most of Europe, but its populations are usually small, scattered, and isolated. ![]() Pseudanodonta complanata inhabits lotic environments, including lowland and mountainous rivers, large drains, and canals (Bonk, 2019 Killeen, Aldridge, & Oliver, 2004) and has also been reported from some lakes (Van Damme, 2011). To date, most of the scientific work on freshwater mussels and the threats to them have been focused on charismatic species such as the freshwater pearl mussel ( Margaritifera margaritifera see e.g., Geist, 2010 Boon et al., 2019), whereas there is less knowledge on the habitat requirements and threats of equally or even more rare species such as the compressed river mussel Pseudanodonta complanata. ![]() 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).
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