Temporal and spatial distribution of glochidial larval stages of European unionid mussels (Mollusca : Unionidae) on host fishes

Glochidia are the larval stage of freshwater unionid mussels that parasitize the fins and gill apparatus of fish. A total of 22 fish species were examined for the presence of glochidia whose distribution on individual hosts was studied on three common fish species, the roach Rutilus rutilus (L.), perch Perca fluviatilis L. and bitterling Rhodeus sericeus (Pallas). Between 1997 and 1999, the fish were obtained from the rivers Morava and Kyjovka and surrounding water pools in the Czech Republic. The glochidia of two genera, Unio and Anodonta, were found. Anodonta glochidia were observed on 10 fish species, Unio glochidia on 17 fish species. There was a difference in spatial distribution of glochidia on the body of the host fish. Unio glochidia were predominantly located on the gills, whereas most Anodonta glochidia were found on the fins, with the highest numbers of glochidia were observed on the margin of the pectoral fins. For the gill apparatus, Unio glochidia were found predominantly on the second and third arch. Anodonta glochidia were predominantly found during winter and spring (November-May), whereas Unio glochidia were more abundant during May and June. The number of glochidia was positively correlated with fish length in perch highly infected by Anodonta glochidia and perch infected by Unio glochidia. Of the three fish species, the highest occurrence of parasites was found on perch with fewer observed on roach. In spite of the close relationship between bitterling and unionid mussels, glochidiosis was rare on this fish species.