Distribution of the alien freshwater snail Ferrissia fragilis ( Tryon , 1863 ) ( Gastropoda : Planorbidae ) in the Czech Republic

We summarize and analyze all known records of the freshwater snail, Ferrissia fragilis (Tryon, 1863) in the Czech Republic. In 1942 this species was found in the Czech Republic for the first time and a total of 155 species records were obtained by the end of 2005. Based on distribution data, we observed the gradual expansion of this gastropod not only in the Elbe Lowland, where its occurrence is concentrated, but also in other regions of the Czech Republic particularly between 2001 and 2005. Information on habitat, altitude and co-occurrence with other molluscs are presented.


Introduction
Probably only one species of the genus Ferrissia (Walker, 1903) occurs in Europe.Different theories exist, about whether it is an indigenous and overlooked taxon or rather a recently introduced species in Europe (Falkner and Proschwitz 1995).This gastropod was most often named as Ferrissia wautieri (Mirolli, 1960) and it was considered as native or cryptogenic, requiring further studies (Glöer 2002, Glöer and Zettler 2004).When referred to as Ferrissia clessiniana (Jickeli, 1882) it was considered as a non-native mollusc in Europe (Falkner and Proschwitz 1995).New studies (Walther et al. 2006) confirmed that the North American ancylid Ferrissia fragilis (Tryon, 1863) is a cryptic invader of European and East Asian freshwater ecosystems.The name F. fragilis is used for all specimens of the genus Ferrissia found in the Czech Republic.

Material and Methods
The datasets used are from senior author's database of over 40,000 records of aquatic molluscs, most of which were obtained by field research during the previous 10 years.The remainder come from Czech museum collections and published papers.
The main sampling method for aquatic molluscs was to wash vegetation or sediments using a metal sieve (kitchen strainer, diameter 20 cm, mesh size 0.5-1 mm).This was combined with a search of various substrates present; stone, wood and artificial surfaces (e.g.plastic bags and bottles).This method is only qualitative -time and area under study was different.All molluscs present in samples were identified.Specimens of Ferrissia fragilis (Figure 1) and most of the other molluscs were identified based on shell morphology.When this identification method was not possible, some species (i.e.Stagnicola, Gyraulus) had to be dissected and then identified using their copulatory organs.The classification used follows Beran (2002).Altogether, 155 records of F. fragilis were obtained; 109 of these included extra environmental data and were used for an analysis of habitat preferences and co-occurrence with other species.The assemblage of the whole site was taken into account for the analysis of cooccurrence, not only the species recorded in the site's part where F. fragilis occurred because these more detailed data was not at disposal.

Distribution in the Czech Republic
In the Czech Republic this snail was recorded for the first time in 1942 from pools near Sadská and for the second time in 1946 from an oxbow lake of the Elbe River near Mělník (Ložek 1971) (Record No. 85 respectively No. 27 in Annex 1, see also Figure 2, A).It is probable that this species was subsequently overlooked because further records were not made until 1975 (Elbe River oxbow lake, near Ostrá, V. Ložek pers.comm.) and 1978 (Ohře River oxbow lake, near Doksany, Flasar 1998) (Figure 2, B).Between 1991 and 2000 the species was recorded at 45 new sites (Figure 2, C).Data obtained until 2001 are summarised in Beran (2002).A rapid increase in records was observed between 2001 and 2005 when 106 finds were recorded (Figure 2, D).This increase was also visible in Figure 3 where the number of records of F. fragilis is weighted by all records of other freshwater molluscs in particular period (data from senior author's database were used).The data represented 0.05 % from 1901-1950, 0.03 % from 1951-1990, 0.34 % from 1991-2000 (Beran 2002), and 0.88 % from 2001-2005.The sites identified for this snail are concentrated in the Elbe Lowland, but between 2001 and 2005 this species was recorded more and more frequently in other regions of the Czech Republic as well (Figure 2).Thus, it is evident that the distribution of this snail is increasing and is spreading despite the possibility that the species was overlooked in the past but at least sampling effort in 1991-2000 was similar to that in 2001-2005.Such a noticeable increase in records is more typical for non-native invasive species e.This species has also been recorded in Czech Republic greenhouses (Horsák et al. 2004) from where it could spread outside, but greenhouse sites are not included in this paper.

Habitats
In all cases this snail was recorded from permanent water bodies; this was in common with Polish records from Bernard (1994) and Strzelec and Lewin (1996).Stagnant waters were a more frequently recorded habitat of this snail than flowing waters.Records from lotic habitats were mainly concentrated in the Elbe River, especially in the stretches upstream of weirs.In fact these stretches had reduced flow due to channelization during weir construction.The most frequently observed habitats were oxbow lakes, pools mostly in floodplains and also water bodies created during mineral extraction (mainly sandpits).Less frequent habitats included water reservoirs.Only occasionally this snail was recorded in rivers and brooks or canals (Figure 4).In terms of substrates, Ferrissia fragilis was usually found on the surface of emergent macrophytes (e.g.reedmace Typha spp., reed sweet grass Glyceria spp.); macrophytes with large floating leaves (e.g. the water lilies Nymphaea spp.andNuphar lutea); different manmade objects (waste, e.g.plastic and glass bottles or jars); wood; dead parts of plants (especially leaves), and also stones.

Altitude
Altitude was available for nearly all the datasets on F. fragilis and these were analysed.In the Czech Republic this snail unambiguously prefers low altitude ranges between 150-250 m (Figure 5).These results correspond with Beran (2002).Lack of data with lower altitude (below 150 m) corresponds with the geographical character of the Czech Republic where sites with altitude below 150 m are restricted.In comparison to previous results of Beran (2002), more data on occurrence at higher altitudes now exist and 552 m is the highest altitude record for this species.

Figure 5 .
Figure 5. Number of records of Ferrissia fragilis in particular altitude ranges

Figure 6 .
Figure 6.Number of molluscs species at particular sites