Recent distribution of Sphaerium nucleus ( Studer , 1820 ) ( Bivalvia : Sphaeriidae ) in the Czech Republic

In the last two decades, research on sibling species complexes has been widely involved in taxonomy and distribution of freshwater molluscs. Some groups of closely related species possess reliable characters only on their reproductive organs, e.g. the structure of male genitalia in “Stagnicola palustris agg.” (O.F. Müller, 1774) (JACKIEWICZ 1993). However, using molecular data one species of this complex – i.e. S. turricula (Held, 1836), previously recognized on the basis of anatomical characters, were found not to be independent from S. palustris s.str. (O.F. Müller, 1774) (BARGUES et al. 2001, 2005). There are also several examples in the case of freshwater clams; the structure of gills and shell porosity were successfully used to distinguish Pisidium globulare (Clessin, 1873) as a separate species from Pisidium casertanum (Poli, 1791) (KORNIUSHIN 1998, 1999). Valid species were recently also found in the “Sphaerium corneum agg.” (Linnaeus, 1758) (KORNIUSHIN 1996, 2001). The taxonomic status of one of these species, i.e. Sphaerium nucleus (Studer, 1820), and its distinctness from the morphologically similar S. corneum (Linnaeus, 1758) has been recently supported by both anatomical and molecular studies (KORNIUSHIN 1996, 2001, KOŘÍNKOVÁ et al. in prep.). Furthermore, these two species differ in their ecology – the former species inhabits almost exclusively small periodical waters and thus its conservation status was a point of discussion. Since S. nucleus was first found at two sites in the Czech Republic in 2003 (KOŘÍNKOVÁ 2006), it was considered likely that many populations from suitable habitats originally determined as S. corneum could in fact belong to S. nucleus. The discovery of new sites for S. nucleus could also be expected. Malacologica Bohemoslovaca (2008), 7: 26–32 ISSN 1336-6939


Introduction
In the last two decades, research on sibling species complexes has been widely involved in taxonomy and distribution of freshwater molluscs.Some groups of closely related species possess reliable characters only on their reproductive organs, e.g. the structure of male genitalia in "Stagnicola palustris agg." (O.F.Müller, 1774) (JACKIE-WICZ 1993).However, using molecular data one species of this complex -i.e. S. turricula (Held, 1836), previously recognized on the basis of anatomical characters, were found not to be independent from S. palustris s.str.(O.F.Müller, 1774) (BARGUES et al. 2001(BARGUES et al. , 2005)).There are also several examples in the case of freshwater clams; the structure of gills and shell porosity were successfully used to distinguish Pisidium globulare (Clessin, 1873) as a separate species from Pisidium casertanum (Poli, 1791) (KORNIUSHIN 1998(KORNIUSHIN , 1999)).Valid species were recently also found in the "Sphaerium corneum agg." (Linnaeus, 1758) (KORNIUSHIN 1996(KORNIUSHIN , 2001)).The taxonomic status of one of these species, i.e.Sphaerium nucleus (Studer, 1820), and its distinctness from the morphologically similar S. corneum (Linnaeus, 1758) has been recently supported by both anatomical and molecular studies (KORNIUSHIN 1996, 2001, KOŘÍNKOVÁ et al. in prep.).Furthermore, these two species differ in their ecology -the former species inhabits almost exclusively small periodical waters and thus its conservation status was a point of discussion.Since S. nucleus was first found at two sites in the Czech Republic in 2003 (KOŘÍNKOVÁ 2006), it was considered likely that many populations from suitable habitats originally determined as S. corneum could in fact belong to S. nucleus.The discovery of new sites for S. nucleus could also be expected.
Malacologica Bohemoslovaca (2008), 7: 26-32 ISSN 1336-6939 The aim of this paper is to summarize all known records of S. nucleus in the Czech Republic based on both results of current field researches and revisions of collection materials.

Material and methods
From 2003 to 2007, living animals and empty shells were collected from the habitats favourable for S. nucleus, using bowl-shaped sieve.The choice of collection sites was made according to the description of habitats published by FALKNER (2000), MILDNER (2001), andKORNIUSHIN (2001).Some populations were also discovered within the scope of other field studies, namely faunistic mapping of molluscs in the Czech Republic.Animals were killed in carbonated water and then conserved in 70% ethanol for anatomical studies.Most of the samples were determined or revised by T. Kořínková.Shape of kidney, shell porosity and arrangement of muscle scars were used as the main identification characters (KOŘÍNKOVÁ 2006 -only specimens bearing at least two of the characters typical for S. nucleus were considered for the study).Other species on the collection sites were also recorded.Available voucher specimens of S. corneum s.lat.deposited in museum or private collections were also re-examined (i.e.collection of J. Brabenec, National Museum in Prague; coll.S. Mácha, Silesian Regional Museum in Opava; coll.M. Horsák in Brno).

Results
In the course of this study 40 recent (i.e.not older than 10 years) records of Sphaerium nucleus have been made in 17 mapping grids of the Czech Republic (Fig. 1).Using only recent records, i.e. those collected by the authors of this paper, we obtained reliable data about the species ecology.The species inhabits presumably small and often periodical pools, littoral zones of ponds, shallow swamps and drains with stagnant water and dense vegetation.The altitude of the species sites ranged from 155 to 442 m a.s.l.(see Fig. 2).The molluscan assemblages of the habitats where S. nucleus was found proved to be relatively consistent in species composition, characterized by Planorbis planorbis (Linnaeus, 1758), Anisus vortex (Linnaeus, 1758), Stagnicola palustris (O.F.Müller, 1774), Planorbarius corneus (Linnaeus, 1758), Valvata cristata (O.F.Müller, 1774), Segmentina nitida (O.F.Müller, 1774), and Bathyomphalus contortus (Linnaeus, 1758), which co-occurred at more than half of the target sites (Fig. 3, Tab.1).Sphaerium nucleus also co-occurred at the majority of recent sites with populations of rare and highly threatened species Anisus vorticulus (Troschel, 1834) (9 sites) and Pisidium pseudosphaerium (Favre, 1927) (10 sites).No recent co-occurrence of S. corneum and S. nucleus in the same habitat has been observed.In a few cases, both species were found in the same sample in the voucher material from studied museum collections.However, those samples could come from fluvial deposits or have been probably collected in two different water bodies lying not far apart.

Discussion
On the basis of recent field researches as well as the revision of voucher museum lots we confirmed that Sphaerium nucleus is distributed throughout the Czech Republic lowlands (Fig. 2).S. nucleus seems to be a typical inha-  x Table 1.List of all species recorded at sites with the occurrence of Sphaerium nucleus (only recent data).Numbers of sites as in Appendix 1, cross -presence of the species.bitant of small stagnant and periodical waters presumably in lowland river alluviums.This is quite in accordance with findings of previous authors (FALKNER 2000, MILDNER 2000, KORNIUSHIN 2001, ZETTLER & GLÖER 2006).Regarding the different habitat preferences of S. nucleus and its ecologically euryvalent sibling species S. corneum, any future findings of both species in the same water body would be of interest, however, in that case, the species could be assumed to inhabit different microhabitats.More attention should be also paid to the possible occurrence of S. ovale, which is not rare in the neighbouring countries -this species rather prefers running waters (ZETTLER & GLÖER 2006, GLÖER 2006, P. GLÖER pers. comm.).The co-occurrence of S. nucleus, Anisus vorticulus, Pisidium pseudosphaerium, and P. globulare has already been reported (FALKNER 2000, KOŘÍNKOVÁ 2006), but the present study gives a more comprehensive overview of the molluscan assemblages in these types of habitats.Bithynia leachii (Sheppard, 1823), Anisus vorticulus, Pisidium pseudosphaerium and P. globulare co-occurring with S. nucleus are classified according to IUCN classification in the Red List of the Czech molluscs (BERAN et al. 2005) as critically endangered (CR).Planorbis carinatus O.F. Müller, 1774 is classified as endangered and Viviparus contectus (Millet, 1813), Anisus spirorbis (Linnaeus, 1758), Segmentina nitida, Anodonta cygnea (Linnaeus, 1758), and Pisidium hibernicum Westerlund, 1894 as vulnerable (VU).S. nucleus itself is considered endangered (EN) by authors of the Red List (BERAN et al. 2005) and this classification is in accordance with our results.We found that the most important threats of this species are destruction of suitable habitats and substantial negative man-made changes (e.g.drainage, eutrophication, succession changes, etc.).

Fig. 1 .
Fig. 1.Distribution of Sphaerium nucleus in the Czech Republic (full dots -recent data from 1996-2007, empty dots -data from revised collections, squares -both recent and historical data).