VARIATIONS OF OPISTHAPTORAL HARD PARTS OF GYRODACTYLUS-SALARIS MALMBERG, 1957 (MONOGENEA, GYRODACTYLIDAE) ON PARR OF ATLANTIC SALMON SALMO-SALAR L IN LABORATORY EXPERIMENTS

On parr of Atlantic salmon Salmo salar L. in laboratory experiments at different water temperatures, opisthaptoral hard parts of Gyrodactylus salaris Malmberg, 1957 differed in size. Their shape, on the other hand, varied only slightly. The opisthaptoral hard parts were largest at the lowest water temperatures and decreased in size with increasing water temperatures (1.5-degrees-C to 20.0-degrees-C). At both 18.0-degrees-C and 20.0-degrees-C the opisthaptoral hard parts of G. salaris were smaller than has been found under natural conditions. In the experiments, the mean size of the opisthaptor in the G. salaris micropopulation at each water temperature gradually increased or decreased compared to the mean size at the start of the experiments. At the end of the experiments the mean size of the opisthaptoral hard parts in the G. salaris micropopulations showed a significant regression to the different water temperatures.