ADAPTATION OF BRACHIONUS CALYCIFLORUS (ROTIFERA) FROM LAKE JINGHU IN SUMMER TO WATER TEMPERATURE: POPULATION GROWTH EXPERIMENT STUDY

To study the population growth rate, average proportion of mictic females, average fertilization rate of mictic females, and resting egg production, Brachionus calyciflorus collected from Lake Jinghu were investigated at four temperatures (20℃, 24℃, 28℃ and 32℃) and three Scenedesmus obliquus densities (1.0106, 2.0106 and 4.0106 cells/mL) by population growth experiments. The results showed that the population growth rate of B. calyciflorus at 28℃ and 32℃ was the highest at all three food levels. At 1.0106 cells/mL of S. obliquus, the average proportion of mictic females at both 20℃ and 24℃ was significantly lower than that at 32℃, but it was not markedly different compared to that at 28℃. The average fertilization rate of mictic females at both 24℃ and 28℃ was the highest, and it was the lowest at 32℃. At 2.0106 cells/mL of S. obliquus, the average proportion of mictic females at 20℃ was the highest. The average proportion of mictic females at 24℃ was lower than that at 32℃, but it was not significantly different compared to that at 28℃. The average fertilization rate of mictic females at 20℃, 24℃ and 28℃ was all higher than that at 32℃. The resting egg production at 20℃ was the biggest. At 4.0106 cells/mL of S. obliquus, both the average proportion of mictic females and the resting egg production at 20℃ were higher than those at any other temperatures. The average fertilization rate of mictic females at both 20℃ and 24℃ was the highest, and it was the lowest at 32℃. Two-way ANOVA analysis indicated that temperature significantly affected population growth rate, average proportion of mictic females, average fertilization rate of mictic females, and resting egg production (P 0.01), food level affected population growth rate and resting egg production (P 0.01), and temperature and food level cooperatively affected average proportion of mictic females and average fertilization rate of mictic females, and resting egg production (P 0.01). These results indicated that the rotifer population adapts high water temperature and water temperature does not directly account for the very low population density of B. calyciflorus in Lake Jinghu in summer, and that the high resting egg production of B. calyciflorus at low temperature may contribute to its reoccurrence in Lake Jinghu during the same period of next year.