Is bitterling (Rhodeus amarus (Bloch, 1782)) threatened by the invasive unionid species Sinanodonta woodiana (Lea, 1834)?

The reproductive strategy of the European bitterling (Rhodeus amarus) involves an obligatory parasitic relationship with freshwater mussels from the family Unionidae. This study sets out to assess the possible threat that the invasive unionid species Sinanodonta woodiana poses to bitterling. The objective was accomplished by analysing the bitterling's reproductive preferences for S. woodiana and four native species of unionid mussels (Anodonta anatina, Unio crassus, U. pictorum and U. tumidus). We performed three experiments in a sand bottom tank. Experiments A and B lasted 20 for days and experiment C lasted 10 days. The highest numbers of eggs and embryos of bitterling in the experiment A were found in A. anatina. On the other hand, U. tumidus had the greatest number of embryos in the most developed embryonic stage (stage VI), followed by U. crassus, U. pictorum and lastly A. anatina, whereas U. crassus had the largest densities of eggs and embryos per unit gill area in cm². As hypothesized, S. woodiana did not contain a single egg or embryo which was also confirmed in the experiments B and C. Results of the experiment C had shown that the first few days bitterlings laid eggs in Unio spp. and only after that in A. anatina. Furthermore, it is evident that bitterlings favour mussels without glochidia. In conclusion, European bitterling will be threatened if an invasive species S. woodiana outcompetes the native unionid species.