Spatial distribution of skin and muscle metacercariae (Digenea) of roach, Rutilus rutilus, and bleak, Alburnus alburnus (Actinopterygii: Cypriniformes: Cyprinidae), from an estuary lake in central Europe

Adult stages of digenean flukes are usually well studied, but their larval forms, the metacercariae, do not receive comparable attention of researchers. As a consequence, metacercariae, especially small ones, are routinely overlooked, despite their serious epidemiological and epizootic potential. Therefore, the existing distribution data are patchy and the biogeographic knowledge is far from being complete. We were not able to trace any papers focusing on the depth of penetration of digenean metacercariae in fish tissues. In this study, we were trying to determine the spatial distribution of metacercariae in the muscles of two freshwater cyprinid fishes, the roach, Rutilus rutilus (Linnaeus, 1758) and the bleak, Alburnus alburnus (Linnaeus, 1758). The study area was Lake Dąbie, a large and shallow lake of the Oder (Odra) River estuary, Poland. We studied a total of 40 roach (12.0–21.5 cm TL 30–120 g TW) and 15 bleak (10.5–14.5 cm TL). The fishes were filleted and each filet was divided into four parts. All muscle tissues were examined for the presence of metacercariae using a trichinoscope and a dissecting microscope. Dorsal muscles of an additional 12 roach were sliced and the depth of metacercariae penetration was assessed. The dominant muscle parasite was Paracoenogonimus ovatus. Metacercariae of other species (Posthodiplostomum cuticula and Apophallus muehlingi) were only sporadically found in muscles roach and bleak (with a preference to the shin). The prevalence of Paracoenogonimus ovatus in roach ranged from 66.7% to 100% and the highest infection intensity was 1973 metacercariae in one fish. The infection parameters differed between length classes.