In the laboratory we examined the effect of pH (5-10 with one interval) on survival, reproduction, egg viability and growth rate (intrinsic growth rate-r m and population growth rate-r) of five Brachionus rotifer species (B. calyciflorus, B. quadridentatus, B. urceolaris, B. patulus and B. angularis). The pH was shown to exert a major influence on egg viability and growth rate (r m and r) for each species. The agespecific survivorship curves within a species were not significantly different at pH 6-10. The optimal pH for each species is near-neutral pH (pH 6-8), and the fecundity decreased as the pH deviated from these values. For each Brachionus species, there was no significant difference between age-specific fecundity curves at pH 7 and 8. At acid pH (pH 5 or 6) higher egg mortality was observed for each species. The r m and population r of the five Brachionus species incubated at different pHs were significantly influenced by pH. The pH supporting the highest r m or r was obtained at pH 6-8, but varied due to species. In this study B. urceolaris and B. patulus could tolerate a broad range of pH, while the populations of B. calyciflorus, B. quadridentatus and B. angulari declined at acid conditions.

In the present study, we evaluated the effects of a graded series of different intensities of predation by predatory rotifer Asplanchna brightwellii (0, 1, 2, 5, 10 and 20 ind. per 50 ml) on the competition outcome between two herbivorous rotifers, Brachionus calyciflorus (196 ± 12 lm in length) and Brachionus patulus (145 ± 8 lm in length), at high (3 9 10 6 cells ml -1 ) and low (0.5 9 10 6 cells ml -1 ) algae food (Chlorella pyrenoidosa) levels. Under low predation intensity (0, 1, 2, 5 predators), B. patulus was able to outcompete and coexist with B. calyciflorus at low and high food level, respectively. Further, increased predation intensity decreased competitive ability of B. patulus at each food level, and it was eliminated from the culture assemblage at high predation intensity (20 predators). However, B. calyciflorus could persist under the highest predation intensity in this experiment. Population growth rate of B. calyciflorus was affected by food level, not by predation intensity, while that of B. patulus showed a reverse result. The maximum population density of B. patulus was significantly influenced by both predation intensity and food level, but for that of B. calyciflorus, food level was the only influence factor. The results suggest that the competition outcome between herbivorous rotifer species with different body sizes are not only affected by food level but also influenced by the predation intensity.

We have described the polymorphism in the hatchlings of resting eggs and the morphological variations between the stem females hatched from resting eggs and their successive parthenogenetic generations in the rotifer Brachionus calyciflorus. We hatched resting eggs of B. calyciflorus in two different culture mediums: unconditioned medium (IOM) and Asplanchna-conditioned (ACM). The hatching rate of resting eggs in IOM and ACM were 32.5% and 28.5%, respectively, and showed no significant difference. Stem females hatching from these resting eggs had three morphotypes (unspined, single short-spined, and two short-spined) and over 80% of these females were spineless ones. Moreover, the frequency of each morphotype stem females showed similar tendency in IOM and ACM. The production of a variety of morphotypes among stem mothers-rather than all unspined as has been previously reported for this species-may be regarded as a form of bet-hedging in this population of B. calyciflorus. Phenotypic changes in morphology between the stem females hatched from resting eggs and their successive parthenogenetic generations in B. calyciflorus were found. The possible mechanisms, responsible for high proportion of spineless phenotype at early generations from resting eggs and increased spined phenotype in successive parthenogenetic generations, were discussed in the article.

This study examined polymorphism in the resting egg size, hatching pattern, morphotype, and starvation tolerance of stem females in a local population of the freshwater rotifer Brachionus calyciflorus. Fifteen rotifer families were cultured under laboratory conditions to induce the production of inbred, homozygous resting eggs. Each family of resting eggs was measured for size and aliquots were then incubated at 10 degrees C or 20 degrees C to test the phenotypic plasticity of hatching dynamics in this population. The distribution of resting egg size was approximately normal in each family, and the sizes of the resting egg shell and embyro showed significant differences among families, ranging from 6.4-8.9 (x 10(5) microm(3)) and 3.7-5.2 (x 10(5) microm(3)), respectively. Hatching rates also varied greatly among families at each temperature, with average values ranging from 10.0-51.5% at 10 degrees C and 3.1- 67.2% at 20 degrees C. Hatching temperature clearly affected the hatching pattern. Resting eggs of most families hatched synchronously at 20 degrees C, whereas they were likely to hatch more sporadically at 10 degrees C. We observed no correlation between size of resting egg and subsequent hatching variability. This was the first time to find a large diversity in morphotype of B. calyciflorus stem females among different families. In some families, most stem females lacked posterolateral spines, but in other families, they had two short posterolateral spines. There was a considerable variation in survival time of stem females that had undergone starvation, and survival time was positively correlated with resting egg size. The divergence in these life history traits may have important implications for understanding the rotifer's adaptive strategy and life history evolution.

In the present work, we tested the hypothesis that the cost of reproduction was evident under stressful conditions with the rotifer Brachionus patulus at different pH levels (5-10 at 1 unit intervals). We used sublethal pH levels (pH 5, 9, and 10) to simulate stressful conditions. We analyzed the correlations between age-specific fecundity (m 1 , m 2 , m 3 , …) versus future survival (l x ? 1 , l x ? 2 , l x ? 3 , … for the entire lifespan) (survival costs) and future expectation of reproduction. . . for the entire lifespan) (reproductive costs), using the data obtained from life table demographic studies of B. patulus under stressful and favorable (pH 6, 7, and 8) pH levels. The results showed that significant negative correlations were observed between age-specific fecundity and future survival and future expectation of reproduction at all tested pH levels, indicating that costs of reproduction exist in the rotifer B. patulus under stressful and favorable pH conditions. However, the percentage of statistically significant negative correlations from total correlations of survival and reproductive costs differed greatly, depending on the tested pH conditions. The percentage of significant negative correlation of reproductive costs is significantly higher under stressful pH conditions (pH 5, 9, and 10) than favorable pH conditions (pH 6, 7, and 8). For survival costs, the same trends are also observed, suggesting that the costs of reproduction were more obvious under stressful pH than favorable pH.