The genetic structure of crucian carps colonies in Ukrainian water bodies was defined by gene marking. The sexual structure and ploidy were investigated, the duration of spawners maturation, life-span, rate of growth and the linear growth of independent species and biotypes were defined. The comparative biological analysis showed that virtually all the researched crucian carps had low linear growth index and early maturation terms. Th e hybrid individuals of C. auratus-C. carassius during the first years of life were characterized by the high rate of growth which was most likely determined by the “hybrid power” which is kept by this biotype only within the starting phase of existence, after which the active growth is significantly reduced. Th e individual biological peculiarities of peculiar species and biotypes of crucian carps do not depend on their genetic nature but in most cases are defined by the external living conditions.
The following proofs of Goldfish more powerful reproductive potential are given: high individual and relative fecundity, adequate fractional spawning process resulting in numerous off spring, represented by different generations of hatchlings, little number of individuals not participating in reproduction, prevailing number of fish eggs in females’ ovaries, larger fraction of impregnated roe. All the above mentioned factors allow claiming that the Prussian carp is being replaced due to its low replacement ability. Perhaps for that reason Carassius gibelio (Bloch, 1782) choose small isolated water bodies, where it makes small populations, backed up by successful spawning with a single male of crucian carp, Carassius carassius (Linnaeus, 1758).
In the upper reaches of the Dnipro it was found a Danube ruffe population. It differs from the population of the middle reaches of the Dnipro by a longer caudal peduncle. The Danube ruffe differs from sympatric populations of the common ruffe by 10 features. For a fast and precise identification of these two species, it was proposed to use the height of the membrane between the first and second rays of the anal fin as a percentageto the length of the body.
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.
Study of population structure of crucian carps in the mixed forests zone of Northern Ukraine (characterized by ample diversity of unregulated aquatic systems) showed the presence of diploid C. carassius and C. auratus, triploid C. gibelio (represented by a series of clones), C. auratus × C. carassius diploid hybrids, and tetraploid hybrids between C. carassius and C. gibelio. The crucian carps of the region compared with those of more southern zones of Ukraine are characterized by numerical prevalence of unisexual form of C. gibelio, and relatively restricted hybridization of C. carassius and C. auratus.