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.