FRESHWATER MOLLUSCS OF THE DYJE ( THAYA ) RIVER AND ITS TRIBUTARIES – THE ROLE OF THESE WATER BODIES IN EXPANSION OF ALIEN SPECIES AND AS A REFUGE FOR ENDANGERED GASTROPODS AND BIVALVES

The malacofauna of the Dyje River, including the lower sections of its five tributaries, is still rich and diversified despite the pollution, regulation and building of several dam reservoirs. In total, 42 freshwater mollusc species (24 gastropods, 18 bivalves) were found at 75 sites. The richest malacofauna (30 species) was recorded in the lower section of the Dyje. Populations of endangered or rare molluscs Theodoxus danubialis, Viviparus acerosus, Lithoglyphus naticoides, Unio crassus, Pseudanodonta complanata, Pisidium amnicum and P. moitessierianum were found. The first three species inhabit only the area under study in the Czech Republic. Especially the occurrence of Unio crassus corresponds with river sections that were only partly affected by human activities (long distance below dam reservoirs, less polluted sections, unregulated watercourses). The occurrence of five invasive species, four originating from other continents (Potamopyrgus antipodarum, Physella acuta, Ferrissia fragilis, Sinanodonta woodiana) and one (Dreissena polymorpha) originally Ponto-Caspian, was confirmed for the Dyje River and its tributaries. The Dyje River formed the main invasion route from the Danube River into Moravia (eastern part of the Czech Republic).


INTRODUCTION
The Dyje (Thaya) River is one of the largest rivers in the Czech Republic and belongs to the Black Sea drainage area.The lower section of the Dyje River has been of interest to malacologists for more than 100 years.The first data are mentioned in ULIÈNÝ (1885), who studied this area in 1882.Published and unpublished data (e.g. material deposited in the National Museum in Prague) till 1998 have been summarised in BERAN & HORSÁK (1998, 1999) who studied aquatic molluscs of the Dolnomoravský úval lowland, including the lower stretch of the Dyje.The macroinvertebrate fauna of the Dyje River downstream of the Nové Mlýny reservoir was studied by HORSÁK (2001).On the other hand, aquatic molluscan fauna of the upper part of the Dyje was poorly known and no published data were available.Little attention was paid to the aquatic molluscs of the lower sections of its tributaries except for the Kyjovka River which was studied by BERAN & HORSÁK (1998).
The Dyje River flows into the Morava River at the Czech-Austrian-Slovak boundary, and both rivers drain a large part of Moravia (eastern part of the Czech Republic).The river was partly changed due to canalisation, dam construction and pollution.To assess how much these changes affected the molluscan assemblages of the Dyje and its tributaries was one of the aims of this research, with special reference to populations of endangered or rare molluscs and, on the other hand, to invasions of non-native gastropods and bivalves.

MATERIAL AND METHODS
The field work was conducted in 2001-2013.In total, 41 sites were sampled in the Dyje River, including the Vìstonice dam reservoir (sites 20-30, one of the five dam reservoirs).The remaining 31 sites were located on five tributaries, and three sites on the canalised Morava River upstream of the inflow of the Dyje River (Fig. 1, Appendix 1).The main sampling method for freshwater molluscs was washing vegetation or sediments on a metal sieve (kitchen strainer, diameter 20 cm, 0.8 mm mesh), combined with collecting by eye (searching of stone, wood and anthropogenic material, e.g.plastic bags and bottles, surface).Unionids were sampled by means of visual inspection of suitable habitats while wading upstream and searching for live molluscs or fresh shells (nacreous layer still lustrous, periostracum coloured).Places with fine sediment were searched by hand.All live individuals of endangered Unio crassus were measured and released.In the case of abundant populations the data were used to study the size structure of the population.
Freshwater molluscs were identified conchologically or, when identification based only on shells was impossible, dissected and determined based on their genitalia.Specimens for dissection were killed in hot water and then fixed in 70% ethanol.No specimen of legally protected species (Unio crassus, U. pictorum, Anodonta cygnea) was killed.Selected material of shells of endangered or rare species is deposited in the author's collection.The classification follows HORSÁK et al. (2010) while WELTER-SCHULTES (2012) was used for zoogeographical classification.where it enters the Dyje River with the average discharge of 1.1 m 3 /sec (VLÈEK 1984).The downstream section, ca.16 km long, was studied.The watercourse was hardly changed and canalised (Fig. 3).

J i h l a v a
The Jihlava River arises near Jihlávka at the altitude of 670 m and after 185 km flows into the Vìstonice dam reservoir at the altitude of 170 m, with the average discharge of 11.8 m 3 /sec (VLÈEK 1984).The river stretch downstream of the inflow of the Oslava River, ca.34 km long, was studied.The watercourse of this section is only partly changed and several weirs were built.

S v r a t k a
The Svratka River arises in the Zïárské vrchy Mts. at the altitude of 760 m and after 174 km flows into the Dyje River at the Vìstonice reservoir at the altitude of 170 m with the average discharge of 27.2 m 3 /sec (VLÈEK 1984).The lower, studied section, ca. 5 km long, was canalised and changed into a straight and slowly flowing canal.

K y j o v k a
A small river originating in the Chøiby upland at the altitude of 512 m; after 86.7 km it flows into the Dyje River at the altitude of 150 m, with the average discharge 1.1 m 3 /sec (VLÈEK 1984).A section about 20 km long was searched.The downstream river stretch approximately 10 km long flows through a floodplain forest and has natural character while the upper studied section was canalised.

MOLLUSCAN ASSEMBLAGES
In total, 42 freshwater mollusc species were found in the Dyje River and the lower sections of its five tributaries: 24 gastropods and 18 bivalves.The Dyje River harbours 41 freshwater molluscs.Fourteen species were recorded in the upper section of the Dyje River (Dyje I, upstream of the Vranov dam reservoir), while in the lower section the molluscan assemblages included from 24-27 (27 in Dyje II; 24 in Dyje III -Vìstonice dam reservoir) to 30 (Dyje IV) species (Fig. 4).The lower section of the Dyje River was the richest studied part while the poorest one was the Zeletavka Brook, with only eight species.The other four tributaries (Jevišovka, Jihlava, Svratka, Kyjovka) were found to hold from 20 to 24 species.In the canalised Morava River above the inflow of the Dyje River only 14 species were recorded.Bithynia tentaculata, Valvata piscinalis, Radix auricularia, Physella acuta, Ancylus fluviatilis, Unio pictorum, U. tumidus, Anodonta anatina, Sphaerium corneum, Pisidium henslowanum, P. supinum, P. subtruncatum were recorded at more than seven studied sections or tributaries while Theodoxus danubialis, Viviparus viviparus, Valvata cristata, Stagnicola palustris, Physa fontinalis, Gyraulus crista, Planorbarius corneus, Ferrissia fragilis, Pisidium amnicum and Dreissena polymorpha occurred in only one or two studied sections.Dreissena bugensis was not found in any studied site.Species which prefer slowly flowing or stagnant water (e.g.Lymnaea stagnalis, Planorbis planorbis, Anisus vortex, P. corneus, Anodonta cygnea) dominated in the Vìstonice dam reservoir which was studied as the only one out of the five reservoirs built on this river.The lists of studied sites and molluscs found at particular localities are shown in Appendices 1 and 2 (Tables 1-5).

ZOOGEOGRAPHICAL ANALYSIS
More than half of the recorded species have Holarctic or Palaearctic distribution ranges (Fig. 5).(Dyje II) upstream of a system of three dam reservoirs (Mušov, Vìstonice, Nové Mlýny) and V. acerosus also in the lower section of the Jevišovka River (Fig. 6).The invasive D. polymorpha was recorded also in the lowest stretch of the Dyje River and in the Kyjovka.

ENDANGERED AND RARE MOLLUSCS
More than 50% of the recorded species are common and widespread, but it is noteworthy that also several molluscs that inhabit the Dyje River and its tributaries are rare and endangered species (Fig. 7).Three prosobranchs (Th.danubialis, V. acerosus, L. naticoides) occur in the Czech Republic only in the studied area.The first one is red-listed (BERAN et al. 2005) as Critically Endangered while the other twoas Endangered.Another endangered mollusc, Unio crassus, was found in the Dyje River and three of its tributaries.Probably abundant populations were recorded in two sections of the Dyje River (Dyje I, Dyje II), scattered populations were found in the Jevišovka and Jihlava, and some specimens were found in the lower section of the Dyje (Dyje IV) and the Zeletavka Brook (Appendix 2: Tables 1-5).An especially abun- formed more than 60% of the population (Fig. 8).
The estimated abundance of Unio crassus in the other sites was much smaller, approximately less than 1 specimen per 1 m of the watercourse.Pseudanodonta complanata, a unionid vulnerable or endangered in many European countries, was occasionally found at several localities in the Dyje River and in only one site of the Morava River.Two endangered pea mussels were recorded only occasionally.Only one specimen of P. amnicum was found at site No. 8 (Dyje II) while P. moitessierianum was found at three sites of the Dyje River (two sites of the Vìstonice reservoir) and two sites of the Jevišovka River (Fig. 9).

NON-NATIVE MOLLUSCS
Four non-native molluscs were recorded in the study area (Fig. 10).P. antipodarum, an invasive spe- cies originating from New Zealand, occurs in the Dyje River except the upper section, and also in the Jevišovka and Jihlava rivers.North American Ph.acuta was found in the Dyje River where it was one of the dominant species in the Vìstonice dam reservoir and was also recorded in all the tributaries except the Zeletavka Brook.Another North American species, F. fragilis, was found in only two sites in the Kyjovka River and in the Vìstonice dam reservoir.S. woodiana, originally distributed in southeastern Asia, was recorded from the lower section of the Dyje River (Dyje IV), Morava River and Kyjovka River.This bivalve was the dominant unionid in several sites (Appendix 2: Tables 1-5).

DISCUSSION
The Dyje River and the lower sections of its tributaries still harbour rich and diverse molluscan assemblages.Although only the main streams of these rivers were studied, the occurrence of 42 freshwater mollusc species (24 gastropods, 18 bivalves) was recorded.This constituted ca.54% of the total freshwater malacofauna of the Czech Republic.The lowest stretch of the Dyje with its 30 species is among the richest Czech rivers, comparable e. g. with the Labe (Elbe) River which is the largest river in the Czech Republic (BERAN 2005(BERAN , 2009)).One of the possible reasons for the diversity is probably the natural character of the watercourse (downstream of Bøeclav), combined with the presence of several species which occur only there in the Czech Republic (Th.danubialis, V. acerosus, L. naticoides, see below) and also the occurrence of invasive non-native species originating from other continents (P.antipodarum, Ph. acuta, F. fragilis, S. woodiana) as well as invasive D. polymorpha, originally a Ponto-Caspian species.This richness is in contrast with the canalised section of the Morava River upstream of the confluence with the Dyje, where only 14 species were recorded.
The comparison with distributional data on freshwater molluscs in the Czech Republic (BERAN 2002) showed that the lower stretch of the Dyje River together with the lower section of the Kyjovka River are the only watercourses with the occurrence of molluscs with Danubial and Pontic distribution ranges (Th.danubialis, V. acerosus, L. naticoides), except the inva- naticoides which occasionally occurred in the Dyje River upstream of the system of three dam reservoirs, and also the population of V. acerosus which was recorded in the lowest section of the Jevišovka River and upstream of these reservoirs (see Fig. 6).The populations of these species are probably relicts that inhabited (together with Th. danubialis) this river section before building of the system of three dam reservoirs (BERAN & HORSÁK 1998).The section of the Dyje around the inflow of the Jihlava and Svratka rivers with its extensive floodplain is among the sites with the highest diversity, not only of freshwater molluscs, and was damaged by construction of the reservoirs between 1969and 1989(BUÈEK 2012)).
The population of Th. danubialis in the Dyje and Kyjovka rivers is situated on the northern edge of the species' range (see e.g.WELTER-SCHULTES 2012).The species is regarded as a Critically Endangered mollusc in several countries, e.g.Austria and Germany (GLÖER 2002, WELTER-SCHULTES 2012), it is endangered in Slovakia (ŠTEFFEK & VAVROVÁ 2006), rare in Hungary (FEHÉR et al. 2004), while in the southern part of its range it is common and widespread.In Croatia it belongs to dominant species, especially in karstic rivers like Korana (BERAN 2013) or Kupa (HADBIJA et al. 1995), where it forms the main part of gastropod biomass.Similarly, the site of V. acerosus is located on the north-western edge of its geographical range (WELTER-SCHULTES 2012), except new introduction(s) in northwestern Europe (e.g.SOES et al. 2009).
Building of the five dam reservoirs, combined with pollution and river canalisation, caused many changes of the Dyje river and also other studied watercourses.The situation of the population of endangered thick-shelled river mussel U. crassus is a good example of how these changes affected the molluscan asemblages of this river.At present this population is divided in two (three) subpopulations, one around the Czech-Austrian boundary in the upper part of the river (sites No. [1][2][3][4]6).This part has a natural character and ends in the Vranov dam reservoir.The second subpopulation (sites No. 16-18) inhabits the short part of the river which has also a relatively natural character and is located about 18 km downstream of the Znojmo dam reservoir.This part is followed by a canalised watercourse and a system of three dam reservoirs without populations of U. crassus.This bivalve was also occassionaly found in the third section of this river, upstream of the inflow to the Morava River.In the past, U. crassus was one of the most common and widespread unionids and was also used as feed for domestic livestock (ULIÈNÝ 1892-1895), so it can be supposed that its occurrence in the Dyje River before anthropogenic changes was nearly continuous while now only isolated and mostly scattered populations live there.Several smaller populations (or only some specimens) found in the tributaries (Zeletavka, Jevišovka, Jihlava) are also isolated from the populations in the Dyje River.The fragmentation of most populations is another negative effect which can cause a decrease or extinction of the thick-shelled river mussel in this region; in Moravia in the Danube drainage area more numerous populations are known only from the studied area and also from the Beèva River, from Malá Beèva canal and from four mill races (BERAN & DOUDA 2009, DOUDA & BERAN 2009).
Like the U. crassus population, populations of another endangered unionid P. complanata are divided into subpopulations.The situation is worse due to the very low density.The scattered occurrence and low densities of this species in many rivers are common in the Czech Republic (BERAN 2002), and make survival of its populations uncertain.
P. amnicum, the biggest pea mussel in the Czech Republic, was found in the Dyje River at site No. 8.The species inhabits unregulated and unpolluted rivers and brooks.In the Czech Republic it belongs to endangered molluscs (BERAN 2002).Unfortunately, only one specimen was found and more detailed research is necessary to confirm the existence of the population.Subfossil shells were found also in the Jevišovka River.In the studied area P. amnicum is known also from a small pond in Vranov (LOZEK & VAŠÁTKO 1997).Other closest sites with the recent occurrence of abundant populations are known from central Moravia in two canals (mill races) of the Beèva River (BERAN 2003(BERAN , 2007)).The last endangered mollusc is P. moitessierianum.The species inhabits slowly flowing waters with muddy sediment; it was found in the Dyje (site No. 18), and surprisingly also in the Vìstonice dam reservoir (site No. 25,26)  The Chinese pond mussel, S. woodiana was first found in the Czech Republic in an oxbow of the Dyje River downstream of Bøeclav in 1996(BERAN 1997).This record is also mentioned in BERAN & HORSÁK (1998) as the only site with the occurrence of this invasive alien species; HORSÁK (2001) did not find it in any of the five studied sites located on the Dyje River downstream of the system of three dam reservoirs.The results confirm the continuing invasion as observed by BERAN (2008a) and show that the Chinese pond mussel has become a dominant bivalve in some sites in the rivers Kyjovka and Dyje.DOUDA et al. (2012) demonstrated that the spread of this alien bivalve was not limited by the availability of hosts and that the temperature conditions in Central European lowland rivers were suitable for completion of its life cycle; they also identified potential threats to native habitats and communities.It is not possible to exclude a negative impact on the populations of common as well as endangered unionids, together with water pollution and habitat alteration.On the other hand, in the lowest section of the Dyje River (site No. 41) where stones are very rare, live specimens and also empty shells of S. woodiana are often used by critically endangered Th. danubialis as suitable microhabitat (Fig. 11).
In comparison with earlier studies (BERAN & HORSÁK 1998, HORSÁK 2001) P. antipodarum was discovered in three studied sections of the Dyje River.This non-native species was first found in the Dyje river basin in the Dyje near Èízov (between sites No. 7 and 8) in 1996 (VOJEN LOZEK, unpublished data).These findings confirmed its rapid expansion also in this river.However, the densities were relatively low and P. antipodarum did not belong among dominant molluscs.The species was also recorded in the Jevišovka and Jihlava but it is impossible to confirm these records with the occurrence in the past because historical data were not available.The presence of P. antipodarum does not seem to have an unequivocally negative effect on the native malacofauna.Another non-native species, F. fragilis, was known only from the floodplain of the Morava River (BERAN & HORSÁK 1998, 2007)   The canal connecting the Danube, Odra and Labe rivers whose building has been discussed for many decades (centuries in the case of the Dunaj-Odra canal) (KRÁTKÝ & LÖW 2005) would significantly increase the possibility of invasion of non-native species, similarly as in the case of many European rivers (BIJ DE VAATE et al. 2002, LEUVEN et al. 2009), as well as consequent changes in the gene pool of autochtonous molluscs.It would also change the character of the lower part of the Dyje River, through significant negative impact to the molluscan assemblages.This would increase the probability of extinction of populations of endangered and rare mollusc species.

CONCLUSIONS
The research on the Dyje River and lower sections of its main tributaries confirmed the existence of rich molluscan assemblages, including three species occurring in the Czech Republic only in this area.The malacofauna of the Dyje River was negatively altered by the construction of several dam reservoirs which divided the river into several sections, changed the watercourses below the dams and in the case of the system of three dam reservoirs (Mušov, Vìstonice, Nové Mlýny) also damaged one of the richest floodplains in the Czech Republic.Other negative effects, such as pollution and regulation, affected also the tributaries.Despite the negative anthropogenic impact, isolated populations of endangered or rare species are still present in several less altered river sections.
The biodiversity of the Dyje River was affected by the occurrence of several non-native aquatic molluscs.The river forms a major route along which alien fauna disperses to the southern part of Moravia.

Fig. 1 .
Fig. 1.The map of the Dyje River and its tributaries with the geographical distribution of the sampling sites.Drawn by MAR-TIN DOLEJŠ

Fig. 4 .Fig. 5 .
Fig. 4. Number of species recorded in particular parts of the Dyje River and its tributaries

Fig. 10 .
Fig. 10.Distribution of non-native molluscs.Drawn by MARTIN DOLEJŠ and in the Jevišovka R.(site No. 49,50).The species was found in the studied area for the first time in 1996(BERAN & HORSÁK 1998) and this record was the first for Moravia (eastern part of the Czech Republic).Finding of an abundant population of V. viviparus in the upper section of the Dyje River (Dyje I, site No. 3-6) between the Czech-Austrian border and the Vranov dam reservoir is noteworthy.The species occurs in the Czech Republic only in Bohemia (western part of the Czech Republic) in the Labe (Elbe) River basin (BERAN 2002) while V. acerosus is known from the Dyje River and in the past also from the Morava River (Danube river basin) (BERAN 2002).The existence of population of V. viviparus is surprising and no information exists on the occurrence of this species in the Dyje River in Austria or the Moravian Dyje River.The Czech section of the Moravian Dyje River was studied (BERAN 2008b) and V. viviparus was not recorded.It is possible that the species was introduced in the Vranov dam reservoir (construction finished in 1934) from the Labe (Elbe) River drainage area in the past (e.g. during building of the dam reservoir).The population lives 115 km away from the nearest population of V. acerosus (site No. 52, see Fig. 1).
and in the Dyje river basin it was first discovered during this research in 2008 at site No. 72.
Fig. 11.Theodoxus danubialis uses living specimens and empty shells of Sinanodonta woodiana in the lowest parts of the Dyje River where stones and other suitable habitats for Th.danubialis are rare.Photo: LUBOŠ BERAN

APPENDIX 2 :
Table 1.List of freshwater molluscs recorded at Dyje I and II study sites Speciesrecorded at the sites is given (only estimation in the case of more abundant species).x -only old shells found APPENDIX 2: Table 2. List of freshwater molluscs recorded at Dyje III and IV study sites Species

Table 1 .
APPENDIX 2: Table3.List of freshwater molluscs recorded at Zeletavka and Jevišovka study sites Species/Site No. 2: Table5.List of freshwater molluscs recorded at Svratka, Kyjovka and Morava study sites APPENDIXFor explanations see Table1.