Changes in the distribution of Anisus vorticulus (Troschel, 1834) (Gastropoda: Planorbidae) in the Czech Republic, monitoring and notes on habitat requirements

The data on the distribution of Anisus vorticulus (Troschel) in the Czech Republic are summarised and analysed, including information on habitats, co-occurrence and monitoring data from selected sites. The species occurs mainly in the floodplains of the biggest Czech rivers. It has always been rare; in the Czech Republic it mostly inhabits small pools and oxbows. Stream regulation, eutrophication and destruction of sites have gradually caused a considerable decline in its population. Selected sites have been monitored since 1999. The highest recorded abundance is 632 specimens per monitoring plot (0.5×0.5 m) and generally fluctuates from 2 to 150 individuals per plot. The highest densities are usually recorded in pools overgrown with Lemna trisulca. A gradual decline or probable extinction have been documented; they mainly result from changes caused by natural succession. Intentional transfers (repatriations, introductions) of A. vorticulus have been tested and the results show that they may constitute a conservation measure.