New record of monogenean parasites on non-indigenous fishes in the Ukrainian Danube Delta

The Danube Delta is a recognised hot-spot for non-indigenous aquatic species, with 11 new species recorded over the last 20 years. The North-American pumpkinseed (Lepomis gibbosus L., 1758), has been common in the Ukrainian Danube Delta since 1918, while the Chinese sleeper (Perccottus glenii Dybowski, 1877) has only recently been introduced. In April 2017, 21 fish species were recorded during a study of the fish fauna around Vylkove (Ukraine, Danube Delta). The Chinese sleeper was the dominant sampled fish from the studied locality, comprising 41.4% of the fish community, while pumpkinseed comprised only 1.1%. Three non-indigenous monogenean parasites were registered: Gyrodactylus perccotti Ergens and Yukhimenko, 1973 infecting the Chinese sleeper; and Gyrodactylus avalonia Hanek and Threlfall, 1969 and Onchocleidus similis Muller, 1936 infecting pumpkinseed. The American monogenean G. avalonia is reported for the first time in Europe though further molecular/genetic characterisation is needed for species validation. Likewise, G. perccotti, a specific parasite of the Chinese sleeper, is recorded in the Danube Delta for the first time. Finally, our data confirm the presence of the monogenean O. similis, a specific parasite of centrarchid fishes, in the Ukrainian part of the Danube.