Growth differences in different biotypes of the hybrid complex of Cobitis elongatoides × Cobitis tanaitica (Actinopterygii: Cypriniformes: Cobitidae) in the Okna River (Danube River basin), Slovakia

Background. Many of Cobitis subpopulations exist as diploid–polyploid hybrid complexes where polyploid gynogenetic females act as “sperm parasites” of diploid individuals. In addition to sexual strategy, the life history of such individuals is also affected by their growth performance. In order to elucidate unknown aspects of the Cobitis hybrid complexes we decided to identify differences in the annual size increments between individuals representing different ploidy and sex, and to compare the condition factor expressed by length–weight (L–W) relation within the same ploidy and sex groups. Materials and methods. In total, 158 specimens representing Cobitis elongatoides (Băcescu et Mayer, 1969) × Cobitis tanaitica (Băcescu et Mayer, 1969) hybrid complex were caught in August and September 2014, of which 132 were analysed. The fish were caught in the Okna River (Danube River basin), Slovakia. Sex was determined based on externally visible morphological features or based on microscopic observation of the gonads. Ploidy level was determined by means of flow cytometry on fin clips taken from each specimen. Age was estimated based on the total number of annuli visible on vertebrae. All statistical models were performed in the R statistical environment. Results. Sex ratio varied within age classes, with 3+ fish primarily composed of females and, 4+ fish mostly triploid. Maximum back-calculated lengths for diploid females were obtained up to 3+, and for the 4+ age group for triploid. L–W relations for adult males and females also differed significantly, but did not vary between diploid and triploid females. There were differences in at specific body lengths for Cobitis with different ploidy level. Diploid males were characterised by short annual age and length increments during the first two years of life. Annual increments in diploid females were highest overall though their frequency decreased after the reaching the fourth year of life (3+). On the contrary, annual increments in triploid females were the lowest within all groups until their fourth year, thought their frequency in the oldest age group (4+). Conclusion. In our study, we first confirmed that yearly increments and maximal age of Cobitis individuals are affected by different ploidy. Contrary, the L–W relations of gutted diploid and triploid females did not differ and they were affected only by sex. We suggest that the  intensity of growth is more affected by ploidy, while the L–W of Cobitis is determined by intensity of feeding and stage of gonad development at the local conditions during season. Therefore, in the future studies of Cobitis growth and condition parameters the ploidy level of individuals should also be considered.