Effects Of Density And Age Of Daphnia Magna On Induced Defense

Induced defenses in plankton can prevent strong population fluctuations in bi-and trtrophic food chains and play an important role in maintaining the stability of community and trophic structure. Inducible colony formation in Scenedesmus can be activated by zooplankton infochemicals; such "Scenedesmus-zooplankton" system has been developed to a well-established model system for studying inducible defenses. There are many reports for inducible defenses, many biotic and abiotic factors all influence the inducible defenses in Scenedesmus. Among these, grazers play an important role in aquatic systems. Measuring the expression degree of inducible defenses of Scenedesmus under different kinds of grazing pressure is also extremely important to understand trade-off strategies of anti-grazing defenses of Scenedesmus. To explore the effects of Daphnia itself on grazer-induced colony formation in Scenedesmus, in this study, we discussed the relationship between intensity of colony formation in Scenedesmus by inducible defenses and different grazing pressure related to grazer different densities and ages (the same density, biomass, feeding intensity).The experimental materials were filtered water from Daphnia magna and Scenedesmus obliquus, and each experiment lasted for a week, we studied grazer different densities and ages, adding D. magna water to S. obliquus, samples were collected then counted using a blood-cell counting chamber under a microscope. We calculated the cell density, the numbers of cells per particle and the mean proportion of cells in different particles, detailed results are as follows:1. Effect of different densities of D. magna on grazer-induced colony formation in S. obliquusWe cultured S. obliquus with filtrate from D. magna with a density of0,6,30,150,300, and600ind L"1, respectively. Results showed no significant differences were detected in growth rates among the treatments with different Daphnia densities. On the day3and5, the proportion of eight-celled colonies in the treatments constituted30%of the S. obliquus populations. Importantly, the number of cells per particle increased significantly with increasing density of Daphnia, indicating a grazer density-dependent response. On the day7and9, the colony size was decreased. It implies that the relationship between the inducible colony size of S. obliquus and Daphnia density may depend on the induction time. 2. Effect of different ages in the same densities of D. magna on grazer-induced colony formation in S. obliquusAt first we used300ind L-1different ages (1,2,3,4,5,7day-old) of D. magna, feeding S. obliquus5.O×105cells mL-1then started the experiement by filtering the water. The result showed that the Daphnia in different ages didn’t influence the normal growth in S. obliquus, there wasn’t a line relation between the colony size in S. obliquus and the body-length in D. magna, we found that the colony had reached the maximum about the5day-old. Then we used300ind L-1different ages (1,2,3,4,5,7day-old) of D. magna, feeding S. obliquusus2.0x106cells mL-1to start experiment (the food was sufficient). The result showed that the D. magna in different ages didn’t influence the normal growth in S. obliquus after changing the food concentration as well, but there is a line relation between the colony size in S. obliquus and the body-length in D. magna.3. Effect of different ages and the same biomass of D. magna on grazer-induced colony formation in S. obliquusAt first we cultured S. obliquus with filtrates from different ages (1,2,3,5day-old) in the same biomass (400,180,70,30ind L-1) of D. magna, the result showed that the D. magna in different ages didn’t influence the normal growth in S. obliquus for the colony formation, except that the significant difference in control and treatment in5day-old of D. magna, other ages of Daphnia had no difference in inducible defense in S. obliquus. Then we cultured S. obliquus with filtrates from different ages (1,3,5day-old) in the same high biomass (1590,280,120ind L-1) of D. magna, the result showed that the colony formation can be strengthened by the filtrate in juvenile Daphnia. However, for the5day-old Daphnia, the colony formation was decreased.4. Effect of different ages and the same feeding intensity of D. magna on grazer-induced colony formation in S. obliquusWe cultured S. obliquus with filtrates from different ages (2,4,6day-old) in the same feeding intensity (2000,500,250ind L-1) of D. magna, the result showed that there were no obvious differences were detected between control and treatments in growth of algae (P>0.05), but colony formation have significant differences between control and any treatment (P<0.05). However, for the different age in every treatment, the ANOVA statistical analysis showed that no significant difference in the size of colony in S. obliquus (P>0.05), it also means that when the D. magna have the same feeding intensity, the effect of different ages of D. magna on grazer-induced colony formation in S. obliquus is the same.