Cyprinus carpio is a commercially i

Cyprinus carpio is a commercially important fish in the aquaculture of the Czech Republic, while Carassius gibelio
is an invasive species combining sexual and gynogenetic reproduction and often entering breeding ponds. The
occurrence of F1 hybrids of these two phylogenetically closely-related fish has recently been reported.
The objective of this study was to investigate selected aspects of physiology, immunity and blood biochemistry
(i.e. parameters reflecting health status and condition) in both parental species and their F1 hybrids, all of the
same age. The morphology was intermediate between parental species.
The majority of biochemical, physiological and immune parameters were primarily affected by sampling period.
For these parameters, hybrids achieved an intermediate character between parental species, but for biochemical
and immune parameters they were still more similar to common carp, i.e. the species in the paternal position. The
intermediate glucose and cholesterol levels together with the intermediate intestine–body size index in hybrids
suggest the role of genetic introgression in determining the range of food utilizable for C. carpio × C. gibelio
hybrids as well as their nutritional status, metabolic activity, and energy intake. In spring, females of gibel carp
and hybrids reached higher GSI when compared to common carp; however, no spring increase in calcium
(important for egg development) was found in hybrid females. Leukocyte count and complement activity in
hybrids were intermediate between parental species. IgM level was significantly higher in females in both
sampling periods, probably reflecting the different costs of reproductive investment between genders. We
propose that the intermediality of traits associated with health and condition status in hybrids resulting from
genetic introgression may potentially represent some advantage for hybrid vigor. Our results also highlight the
good condition status of C. gibelio and may indicate a potential risk of competition between this invasive species
and the economically important C. carpio in breeding ponds.