Distribution of Anodonta (Sinanodonta) woodiana (Rea, 1834) in inland waters of Serbia

Aquatic biotopes are, due to their unique features, among the most disposed ecosystems to bioinvasions. Serbian waters are not an exception, with a increasing colonization by allochthonous organisms observed there during the last few decades. Nonindigenous aquatic species were found among plants, vertebrates and invertebrates. One of the most invasive aquatic macroinvertebrate species found in the region is the Chinese pond mussel Anodonta (Sinanodonta) woodiana (Rea, 1834). The species is dispersed along lowland rivers, associated wetlands and manmade canals. Heavily modified and artificial aquatic habitats, with high silting rates, were found to be especially suitable for population by A. woodiana. A mass occurrence of Chinese mussel was observed among these habitats, particularly where bottom substrata were characterised by the domination of silt-clay.

Rapid spread, frequent findings and mass occurrence of A. woodiana were reported from several recipient areas in the Danube Basin -Slovakia (Halgoš 1999), Hungary (VITUKI 2001) and Serbia (Paunovic et al. 2005a). Recently, the species has been reported as frequent along the Middle Danube (ICPDR 2002).
The aim of this paper is to present the current distribution of Chinese pond mussels in Serbian waters. Investigations on the distribution of Chinese pond mussel should be continued, due to the invasive character of A. woodiana, its mass occurrence in the region, as well as possible impact to the autochthonous bivalves via competition (Essl and Rabitch 2002).

Materials and Methods
Special attention has been focused on collecting data on the distribution of non-native fauna within Serbian waters during macroinvertebrate surveys performed in the period 1998-2006. These investigations comprehended transboundary rivers (sectors of the Danube which are shared with Croatia and Romania), as well as the bordering region with Hungary (the Danube and the Tisza River). Along with other information, datasets on the distribution of A. woodiana were gathered during these surveys.
Benthic samples were collected by various techniques -benthic hand nets, hydraulic polyp sampler, Van Veen (0.027 m 2 ) and Eckman (0.0225 m 2 ) grabs and benthological dredge. Diving was also performed to collect mussels. The mussels were gathered in periods of low water conditions at depths between 0.4 and 7 m.
The coordinates of the sampling points were measured by GPS ("Garmin Etrex") and charted by using ArcView software (map 1:300,000, system WGS_1984).
For some sampling sites, the abundance of mussels has been evaluated by collecting and counting individuals within randomly selected square areas of 0.5 m 2 by diving. A diver sampled all mussel specimens from chosen quadrant and removed it to the shore for identification.

Results and Discussion
We are presenting the findings of A. woodiana According to Csányi et al. (2005) the Chinese pond mussel has a dense population in the Danube River, at the inflowing stretch of the Iron Gate Reservoir. Our investigations confirmed that  During our investigations, the Chinese pond mussel was observed within habitats with a predominantly silt-clay substrate, together with native mussel species -Unio pictorum (Linnaeus, 1758), U. crassus (Philipsson, 1788), U. tumidus (Philipsson, 1788), Pseudanodonta complanata (Romässler, 1835) and Anodonta anatina (Linnaeus, 1758), as well as other molluscan species -Viviparus acerosus (Bourguignat, 1862) and Viviparus viviparus (Linnaeus, 1758). In some cases, A. woodiana was found to be the dominant mussel (Figure 2, Annex,Records No. 10,26,40,42 and 43). Thus, at the Velika Morava River (Figure 2, Annex, Record No. 43) A. woodiana was found together with U. pictorum in a ratio of 5:1 specimens, respectively. During the 2003 Tisza expedition (August 25-29), we had the opportunity to count mussels along the bank region of the Begej River due to an extremely low water level (Figure 2, Annex,Record No. 40). At this location, an abundant population of A. woodiana was observed, with a mean number of six individuals per square meter; compared to all other mussel taxa confirmed, A. woodiana dominated by a 2:1 ratio (Figure 3). Observed domination of A. woodiana in comparison with native mussel taxa infers that Chinese pond mussel could have an impact on autochthonous bivalves via competition (Essl and Rabitch 2002). The introduction and spread of the Chinese pond mussel seems to be closely correlated with the introduction of the Grass Carp Ctenopharyngodon idella (Valenciennes, 1844), the Prussian Carp Carassius auratus gibelio Bloch, 1783, the Silver Carp Hypophthalmichthys molitrix (Valenciennes, 1844) and the Bighead Carp Arstichthys nobilis Richardson, 1844 from China and other Far East countries into Serbian waters and neighboring regions. These species, belonging to the so-called Chinese complex, were introduced into Serbian waters in the sixties and mid-seventies (Cakic and Hristic 1987) of the XX century, which suggests that the Chinese pond mussel was introduced at the same time. Rapid spread and mass occurrence of A. woodiana have been reported from several recipient areas in the Danube Basin -Slovakia (Halgoš 1999), Hungary (VITUKI 2001) and Serbia (Paunovic et al. 2005a). Stefek et al. (2004) reported that, since 1999, A. woodiana has been expanding from the Danube River to other larger rivers in Slovakia. Our investigations indicated a similar trend -the Chinese pond mussel has been found in the downstream stretches of the Sava and the Velika Morava River, which suggests that it has spreads to those rivers from the Danube. With regard to the Tisza River, the situation is not as clear, because the Chinese pond mussel has been found 419km upstream of its confluence with the Danube at Kisköre Reservoir -Hungarian part of the Tisza River (Csányi 2002). This situation could indicate that A. woodiana has expanded into the Tisza River not only from the Danube River, but also from other sources in the Middle Stretchvia the introduction of fish species from China and Far East.
Recent international surveys (ICPDR 2002) indicated that the Chinese pond mussel is dispersing from the middle Danube to the upper and the lower stretches. The spread of nonindigenous species along the Danube (in both directions, upstream and downstream), as well as the expansion of neobiota from the Danube to its tributaries, indicates that the Danube River is an important invasion corridor (lower part of the so called "Southern Invasion Corridor" sensu Bij de Vaate et al. (2002)).
The growing colonization of allochthonous organisms in Serbian waters has already been observed and aquatic alien species have been found among plants, vertebrates and invertebrates (literature review in Paunovic et al. 2004Paunovic et al. , 2005b. Large lowland rivers in the region of the Middle Danube are under particular pressure by neobiota , Simonovic et al. 1998, 2001, Csányi 1999, 2005aPaunovic 2004, Pavlovic et al. 2006). This has been confirmed by the distribution of A. woodiana in this research.
Further, investigations on the distribution of A. woodiana, as well as on other aquatic neobiota in Serbia, are needed in order to provide effective prevention measures for the introduction and dispersal of invasive species.