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Wild-captured 16 diploid, five triploid and one tetraploid Prussian carp Carassius gibelio males produced motile haploid, aneuploid (1·5n) and haploid to aneuploid (> 2n) sperm, respectively, in similar concentration and with the lowest percentage of live spermatozoa in sperm for the tetraploid male (mean ±s.d. 83·03 ± 1·76%; P 0·05).

Morphometric and genetic variation were examined in parental Carassius auratus gibelio and Carassius carassius individuals, and their natural hybrids. Of meristic traits, only the number of gill rakers clearly distinguished hybrids (39·4 ± 1·4) from parental C. a. gibelio (48·3 ± 0·7) and C. carassius (28·6 ± 1·1). MtDNA sequences showed that the hybrids were descendants of female C. a. gibelio. Microsatellite analysis confirmed the presence in hybrids of variants typical of C. a. gibelio and C. carassius.

A reduction of epidermal club cells and an increase of goblet cells were found in Carassius gibelio during spawning when compared to postspawning. A significantly lower proportion of club cells at spawning was found in diploid males and triploid females than in diploid females. It could be linked to male efforts to avoid a fright reaction and the potential adoption of this strategy by gynogenetic females, or alternatively to a higher parasite infection or immunosuppression during spawning.

Morphological (meristic and morphometric traits), karyological and molecular (microsatellites, cytochrome b) analyses were performed to characterize a hybrid of the roach Rutilus rutilus and nase Chondrostoma nasus. Meristic and morphometric traits were different between hybrid and both parental species. The number of chromosomes found in hybrid specimen indicated that this individual represents the post-F(1) generation of hybrids and the microsatellite analysis of the hybrid showed the presence of variants typical for R. rutilus and C. nasus.