PISIDIUM PSEUDOSPHAERIUM FAVRE , 1927 ( BIVALVIA : SPHAERIIDAE ) IN THE CZECH REPUBLIC – RARE OR OVERLOOKED ?

The data on the distribution of Pisidium pseudosphaerium in the Czech Republic were summarised and analysed in an attempt to answer the question of whether the species was really very rare or only overlooked. P. pseudosphaerium was recorded at 17 sites, most of them found in the last 15–20 years, while earlier records were very rare. Very abundant populations (metapopulations) were found in eight sites, while the abundance in the other sites was much lower. P. pseudosphaerium is a vulnerable species inhabiting natural or only slightly altered sites (now often within protected areas); disappearance of such sites from most of the Czech Republic is the probable reason for its rarity.


INTRODUCTION
The False orb pea mussel Pisidium pseudosphaerium, a probably Europaean species (weLter-schuLtes 2012), is listed as critically endangered in the Czech Republic (beran et al. 2005) and regarded as very rare there (beran 2002).Recent data on its occur-rence made it possible to summarise and analyse its occurrence in the Czech Republic in an attempt to answer the question of whether the species is really very rare or only overlooked.

MATERIAL AND METHODS
The data used here come from the author's database containing more than 57,000 records of aquatic molluscs in the Czech Republic, obtained from his own field research since 1993, from many published and unpublished papers, as well as from private and museum collections since 1825.Most of the records of P. pseudosphaerium since 1995 were acquired during the author's field research, some recent data come from MichaL horsáK's collection.

RESULTS
P. pseudosphaerium (Fig. 1) was discovered in the Czech Republic much later than most of other native freshwater molluscs.The species was not mentioned in the old monographs of Czech molluscs (sLavíK 1868, Uličný 1892-95), since it was described in 1927; likewise there is no mention of this species in ložek (1948,1956).The first records of this pea mussel refer to the Bohdanečský rybník Pond (1959), and ponds in the U Houkvice Nature Reserve (1961), both in Eastern Bohemia (brabenec 1978).Only 11 records from six sites were made till 2000 (beran 2002), and more than 100 records from 16 sites became available since 2001 (site here means a more extensive area with the occurrence of this speciese.g.system of pools, extensive pond and wetlands in its surroundings) (Fig. 2).Most sites are situated at altitudes of 150 to 300 m, and only one at 375 m.At four sites P. pseudosphaerium was recorded sporadically, or only a few individuals were found.In two sites it probably became extinct during the last 10-15 years.More abundant populations, but with specimens usually found at only one or a few sampling points, were found in five sites while eight sites held very abundant populations (metapopulations), with individuals recorded at many sampling points.These important sites with the most abundant populations are shown in Fig. 3 and described in Appendix 1.The highest recorded abundance was estimated around 2,000 ind./m 2 .
In nearly all cases the species occurred in shallow and overgrown parts of ponds, oxbows, pools, wetlands or similar shallow water bodies (Figs 4-6

DISCUSSION
Although the species was regarded as extremely rare, the more intense research in the last decade(s) revealed more sites than was expected.On the other hand, most of the sites are located in most valuable areas which are now strictly protected (e.g.national nature reserves, national nature monuments) or at least within protected landscape areas.P. pseudo sphaerium is a vulnerable species inhabiting natural or only slightly altered sites, which have largely disappeared from most of the Czech Republic.Many sites (especially lowland sites with small populations) are threatened by natural succession, eutrophication and destruction; the species has probably disappeared from two sites.
In the Czech Republic, the species is classified as Critically Endangered and the results of this study confirm its status.The situation is similar in many European countries, as shown by Fehér et al. (2004)   The results of this study show that P. pseudosphae rium has truly been overlooked in the past, however, despite the numerous sites found in the last 20 years, it should still be considered as a rare species inhabiting natural or only slightly altered sites which often have a relict character.It is possible that new sites will be found in the future but probably only in well preserved areas.
Fig. 2. Distribution of Pisidium pseudosphaerium in the Czech Republic.Drawn by J. vrba