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The ichthyofauna of the Black Sea is comparatively poor and includes about 200 species (Boltacev 2003). The gobiid fishes (Actinopterygii: Gobiidae) are a large group of approximated by 28 species inhabiting coastal habitats of the Black Sea (Miller 1986, Smirnov 1986, Rass 1993). Five Black-Sea gobiid species are important commercially (Smirnov 1986) and they have also been targeted by anglers. In the northwestern Black Sea (NWBS), gobiids can be found in different salinities ranging from fresh water (0.5‰ salinity off the Dniester River mouth) to full-salinity sea water (30‰-35‰ in the Tuzly group of lagoons). These fish make up two groups according to their origin: 1) Ponto-Caspian relict species that are endemics of the Sarmatic and Pontic basins; typical representatives of this group are: Mesogobius batrachocephalus (Pallas, 1811);

The known metazoa parasite fauna of the invasive round goby Apollonia melanostoma (formerly Neogobius melanostomus) consists of 12 species. The core of the parasite fauna comprises two species: Cryptocotyle concavum and Diplostomum spathaceum; secondary species are absent; satellite species include Cercariae gen. sp. and Ergasilus sieboldi; rare species are Acanthocephalus lucii, Anguillicola crassus, Bothriocephalus sp., Dichelyne minutus, Hysterothylacium aduncum, Pomphorhynchus laevis, Piscicola geometra, and Tylodelphys clavata. Fifty percent of metazoa parasites that occurred in the invasive round goby in the Gulf of Gdańsk (an invasion that was first detected in 1990) are also typically found in the native Gulf of Gdańsk gobiids. The round goby hosts common fish parasite species: C. concavum and D. minutus, but none that are unique to the species and no Ponto-Caspian parasites. Notably, the parasite species of the invasive round goby in the Gulf of Gdańsk includes species that are atypical for this species in its native habitat. In its new habitat, the round goby variously serves the roles of definitive, second intermediate, and paratenic host for different parasite species. The fish species is involved in a parasitic system that includes fish-eating birds, fishes of different ecological groups (predatory, planktivorous, and benthivorous), and invertebrates.