Co-occurrence of Sinanodonta woodiana with native Unionidae in the lower Oder

Sinanodonta woodiana (Lea, 1834), known as the Chinese pond mussel, originates from Asia but has been spreading all over Europe since the early 1980s, also in Polish waters. In Poland, it was recorded in at least 25 localities (Urbańska & Andrzejewski 2014), and the number of localities has been increasing as evidenced by numerous reports (Gąbka et al. 2007; Ożgo et al. 2010; Najberek et al. 2011; Urbańska et al. 2012; Andrzejewski et al. 2013; Urbańska et al. 2013; Spyra et al. 2012; Domagała et al. 2013; Soroka et al. 2014). In Poland, apart from lakes, the species was found primarily in fish ponds (Mizera & Urbańska 2003; Andrzejewski et al. 2013). In other European countries, however, its numerous populations often colonize rivers and lakes (Popa et al. 2007; Pou-Rovira et al. 2009; Lajrner & Crnčan 2011; Douda et al. 2012; Beran 2013; Colomba et al. 2013; Kamburska et al. 2013; Guarneri et al. 2014; Horáčková et al. 2014; Beran 2015).

Scanty information on the presence of S. woodiana in the Polish lotic waters applies to the Narew River (Böhme 1998). Moreover, the species was also reported in a discharge canal (so-called Thermal Canal) with heated water from a power plant, in canals of the Oder (Domagała et al. 2007), as well as in the Postomia River (Domagała et al. 2013). Information on the malacofauna of the Oder is presented in numerous publications (e.g. Domagała et al. 2004; Łabęcka et al. 2005; Piechocki & Szlauer-Łukaszewska 2013). Mollusks of the Oder have been studied since the 1990s and any information on the presence of S. woodiana in the main course of the Oder has been reported. This paper is the first report on the colonization of a river by S. woodiana in Poland. It also presents a brief description of the unionid community that co-exists with this invasive species.

On the 7^th October 2015, we sampled the waters of the lower Oder. Four sampling sites were selected: (1) at the confluence of the Warta and the Oder; (2) on the Oder, at 618 km from the river’s source; (3) in an open oxbow lake of the Oder; and (4) on the Oder, 624 km from the river’s source (Figure 1). At sampling sites 1, 2, and 4, samples were collected between groynes. At each site, 6-10 plots (0.25 m² each) were randomly designated and searched manually with the use of a square frame. Due to the fact that the water level was extremely low in that period (23 cm in 2015, i.e. more than 200 cm lower compared to 2014), we were able to search the bottom directly to a depth of 80 cm. We estimated the density (D) and frequency (F) of the collected S. woodiana specimens, as well as we made basic biometric measurements and identified the coexisting bivalve species.

Figure 1

In total, 18 individuals of this species were found at all sampling sites. Their averaged biometric data are presented in Table 1. Their length and mass varied considerably. Apart from the live specimens, also numerous empty shells of this species were found at all the sites.

Table 1

Biometric data of Sinanodonta woodiana in the lower Oder River

The estimated densities of S. woodiana at the studied sites varied significantly but were generally low (Table 1). The lowest density of 0.3 ind. m^-2 was recorded in the oxbow lake of the Oder (site 3), while the highest one of 2.6 ind. m^-2 – at 624 km from the river’s source (site 4) (Figure 2). The distribution of this species was uneven at all the sites, with frequencies ranging from 16 to 25%.

Figure 2

In the collected samples, S. woodiana coexisted with native species of Unionidae: Anodonta anatina, Unio pictorum, U. tumidus, and Pseudanodonta complanata. Moreover, some samples included infrequent specimens of Dreissena polymorpha (Dreissenidae), while another alien and invasive species – Corbicula fluminea (Cyrenidae) – was found near the main current of the Oder. U. tumidus was the dominant species at two sites (2 and 3), A. anatina – at one site (1), while two species co-dominated at site 4: U. tumidus and A. anatina. P. complanata was an accessory species (the only one less numerous than S. woodiana), present at three sites (1, 2 and 4). Densities of Unionidae varied significantly between samples within the investigated sites. The maximum density was 516 ind. m^-2 at site 2, and the average values varied from 37.3 in the oxbow lake (site 3) to 98.3 ind. m^-2 in the main current of the Oder (site 2).

The discovery of S. woodiana in the Oder and the Warta confirms the suggestions of Domagała et al. (2013) who, after finding live specimens in the Postomia River, hypothesized that the occurrence of this species in the Warta was only a matter of time or had already happened and required confirmation. The cited authors also emphasized that it is necessary to thoroughly examine these issues.

The lowest density in our study was recorded in the oxbow lake of the Oder, but according to Zając et al. (2013) – who found S. woodiana in oxbow lakes of the Vistula together with abundant native malacofauna (Anodonta anatina and Unio pictorum) – such habitats may be equally important to Unionidae. Our findings also indicate yet another route of expansion to local ecosystems and the tolerance of S. woodiana to adverse environmental conditions, i.e. colder waters in summer (as compared to fish ponds) and partial freezing in winter.

Our results differ from the reports of Douda et al. (2012) and Beran (2013), who found that S. woodiana is a dominant species at some sites in Czech rivers (e.g. Kyjovka and Dyje), which indicates that the species finds favorable conditions in rivers of Central Europe, so it may pose a threat to the populations of native species. Currently, native species are most abundant in the Oder – mainly U. tumidus and A. anatina – but changes similar to those observed in Czech rivers cannot be excluded, considering the observations made in Lake Balaton (Benkő-Kiss et al. 2013), which show changes in unionid communities, indicating that S. woodiana considerably increased its density and biomass, and gradually replaced the co-existent A. cygnaea. Because of the local populations of S. woodiana in rivers, the species can spread even more effectively, but there are still no clear answers to questions concerning its impact on new habitats, especially on native species of Unionidae. The results of this study indicate that S. woodiana is able to colonize a large lowland river in Poland, but so far has not dominated this ecosystem.

The impact of this alien species on the native ones was too short, so it is impossible to provide any evidence or to evaluate the effects of S. woodiana on native species of mussels. To confirm the impact of S. woodiana, the long-term research and regular monitoring are needed. They have been performed at many sites in Poland, including new habitats and locations.