Background. Ruffe, Gymnocephalus cernuus is a small fish inhabiting vast areas of Europe, Asia, and also North America. Its substantial geographical dispersal range in different climatic zones, in waters of different parameters indicates that this is a particularly plastic species. Recent implementation of new fish-processing machinery has increased dramatically the economical importance of ruffe, which is now treated as a gourmet food. In the wake of the increased interest in this fish we decided to study aspects of its developmental biology. Methods. Fertilised eggs and the developing embryos of ruffe were incubated under five different temperature regimes. They were monitored in two planes, horizontal and vertical, using a light microscope fitted with a digital camera and connected to a computer with a monitor and a VCR. The data collected were processed using MultiScan software.Materials and methods.Results. Perivitelline space in ruffe constitutes as much 65% of the egg volume. This facilitates movements of an embryo and enhances processes of gas exchange through mixing of perivitelline fluid. It finally contributes to the distribution of the hatching enzyme on the inner surface of the egg shell. Similarly as in the other percid fish, the lack of integration between the "lipid raft" and the embryonic disc, causes the fall of the disc on the lateral side of the vitelline sphere. This lateral position persists throughout all sequential stages of the embryogenesis. The diversified thermal regimes of the development affect the timing of the embryogenesis, as well as the condition, weight, and the size of newly-hatched ruffe larvae. The highest rate of specimens hatched in good condition-marked by the highest weight and body length-was observed at the optimal temperature range of 16-18oC. Newly-hatched larva is small (5 mm body length, 0.10 mm3 of yolk sac volume), active, and fully prepared for independent life in the water. Under the optimal temperature range, determined in the present study, the duration of embryonic development of ruffe (from fertilisation to hatching) was 2122 dh (accumulated degree-hours).Conclusion. The present study constitutes a contribution to the knowledge of the embryonic development of ruffe and its biological sense, emphasizing the morpho-physiological differences between this species-living under, specific, diversified ecological conditions-and other fish.

fish, ruffe, Gymnocephalus cernuus, reproduction, egg, embryogenesis, embryonic motorics, temperature