The analysis of present-day crucians’ settlements in water systems of Eastern Ukraine designated the predominance of the digeneous Goldfish, C. auratus, in the region, the number of which made 78.7 %, from the total number of the examined representatives of the genus. The second group consists of gynogenetic Prussian carps, C. gibelio (14.3 %); it is represented by the clone biotype and recombinant individuals. Crucian carp, C. carassius (3.6 %), turned out to be rare and its number did not exceed the number of the caught hybrids C. auratus × C. carassius (3.4 %). The retrospective analysis of literature data and museum collections gave an opportunity to describe the changes in species composition of the genus which took place during the last 150 years. Within this period the crucian carp, which used to be the single and most common representative of genus Carassius (Jarocki, 1822) in the region, became nearly an extinct species. In the meanwhile the representatives of the group of species of Prussian carps, C. auratus + C. gibelio, which appeared in the region in the late 1960s, rapidly increased their number and became the most numerous fish of the Eastern Ukraine. The discovered tendency is not unique for the researched region; in general it reflects the European tendency for the crucian species. The reasons for that are rivers’ regulation and destruction of bottomland ecosystems. The secondary factors for the elimination of C. carassius are the competitive relations of individuals representing both species and easy hybridization, during which the more numerous species C. auratus absorbs the rare C. carassius.
The article presents the original findings of Paracoenogonimus ovatus (Katsurada,1914) in fish in the Southern Bug and the Inhul rivers in Mykolaiv Region in 2012-2014. The roach (Rutilus rutilus), silver bream (Blicca bjoerkna), bream (Abramis brama), crucian (Carassius gibelio), rudd (Scardinius erythrophthalmus), pike (Esox lucius), pikeperch (Sander lucioperca) were found to be infected with metacercariae of P. ovatus. Metacercariae of P. ovatus are described based on the original material. The highest prevalence of infection was observed in the roach, 82.3 %, the pike was infected in the less degree, with prevalence of 15.3 %. The infection intensity was highest in the roach, up to 247 specimens; that of the pikeperch was the lowest - 17 specimens. The highest occurrence of P. ovatus was observed in the parts of the river Southern Bug in Zhovtnevy, Mykolaiv, Novoodesky Districts, whereas in Voznesenky, Pervomaysky Discticts and in the city of Mykolaiv the occurrence was the lowest. In the area of the river Inhul the parasite was observed mostly in the waters of Bashtansky District in Mykolaiv Region.