Dr. Éva Murai szerk.: Miscellanea Zoologica Hungarica 4. 1987 (Budapest, 1987)
Kiss, I.: Ecological studies on zooplankton species important for fish farms. II. Interspecific relations between zooplankton species
The Cladocera spp. react to the attack by the predators (planktonic species or fish) by changing the proportion of their bodily parts in size (LYNCH, 1977; DODSON, 1974). Cyclomorphosis serves as a means of defense against being taken as a prey by fishes. The selective consumption of fishes results in the evolutionary change of the cyclomorphosis of the plankton members (JACOBS, 1977). In fresh-water lakes the interactions between the Copepoda and Cladocera spp. appear in an extremely complex form both morphologically and ethologically. For instance in the case of Bosmina the lengthening of the antennae and spines is accompanied by the thicken, ing of the endoskeleton, which ensures protection against the copepods. There is protection in the sphere of multiplication too, as the larger eggs with more yolk lead to a shorter period of embryonic development (KERFOOT and PETERSON, 1979). It is has been long known that in the case of Brachionus calyciflorus, a modification of spine-length can be observed. It has been proved by experiments that the increase in spine is induced by external factors, the most influential of which is the presence of the predator Asplanchna spp. in the given water habitat. The peptide derivative discharged by the As planchna into the medium enters the body of Brachionus calyciflorus, and induces the encrease of the spine-length of the off-springs by influencing the ova in the female' s ovary. The increase of the spine-length of the new-born animals reflects the last day's population density of Asplanchna (HALBACH, 1970). This correlation is true only for the new-born, since the older animals are not influenced by the induction of the Asplanchna. This compound is permanently produced and discharged into the medium by the As planchna. However, if they die out in a water habitat, this compound disappears very quickly (in one or two days), so it has no effect on the new-born Brachionus (HALBACH, 1971). The lengthened spine, which can be stiffened, provide the Brachionus calyciilorus with an effective device for protection against being swallowed by the predator Asplanchna. The larger the spines are the safer the protection against the predators. Even in the> case of very young and small individuals of Brachionus calyciflorus, the advantage of the spined animals can be proved (HALBACH, 1979). MATERIALS AND METHODS 2 We made our investigations in the 100 m fish-progeny breeding ponds on the Tempered Watered Fish-breeding Farm of Százhalombatta. Using the selective plankton preparation method (TAMÁS and HORVÁTH, 1975; TÖLG, 1981) we created pure Rotatoria breeds, in which the number of the Brachionus calyciflorus (Pallas), Asplanchna priodonta (Gosse), Asplanchna brightwelli (Gosse) species was overwhelming. Using the above-mentioned method we also created pure breeds in the case of two Cladocera species (Moina brachiata/ Jurine/, Daphnia magna Straus). The detailed description of the methods is given in KISS (1985). In the present work we used the data of the underlisted experimental lakes (the starting date of the experiments means the filling up of the lakes): Moina brachiata breed: I. 15. July, 1975. - 11. August, 1975. (15 samples in 27 days) II. 8. August,1975. - 8. September, 1975. (14 samples in 31 days) Daphnia magna breed: I. 20. July, 1975. - 22.August, 1975. (16 samples in 32 days) II. 8. August, 1975.- 8. September, 1975. (13 samples in 31 days) Rotatoria breed: I-IV. 22.September, 1976-25. October, 1976. (10 samples in 34 days) The Moina1, -II and Daphnia-1, -II breeds are identical with the ones analysed in detail in KISS (1985). In addition to following closely the number of individuals of the Rotatoria species, it was also necessary to take the measurements of the postero-lateral spine (PL) and thelorica length (L) of the individuals of Brachionus calyciflorus. We took the measurements of 45-50
