食物浓度对方形臂尾轮虫发育历期与实验种群增长参数的影响

为探讨富营养化水体中大量发生的轮虫与微藻之间的关系 ,在实验室条件下 ,观测了不同藻类食物浓度下方形臂尾轮虫生命周期中各发育阶段的历时以及种群增长参数的变化。实验选用蛋白核小球藻为食物 ,浓度梯度为 0 1× 10 6、 0 5× 10 6、 1 0× 10 6、 2 0× 10 6、 4 0× 10 6、 8 0× 10 6、 12× 10 6cells/ml。结果表明 :食物浓度对方形臂尾轮虫的胚胎发育时间与繁殖前期历时、产卵量、世代净生殖率、世代时间和种群的内禀增长率均有明显影响。在食物浓度为 2 0× 10 6cells/ml时 ,方形臂尾轮虫的繁殖前期最短 ,产卵量最大 ,世代净生殖率和种群增长率最高 ,世代时间最短。方形臂尾轮虫可在高于其最适食物浓度的较宽的食物浓度范围内 ,保持较高的种群增长率 ,表现出较强的对环境中高浓度藻类的适应     

食物种类和浓度对红臂尾轮虫种群增长、个体大小及卵大小的影响

采用群体累积培养法,研究了藻类食物的种类和浓度对红臂尾轮虫种群增长、个体大小及卵大小的影响．结果表明,藻种类和浓度对红臂尾轮虫种群增长率、个体体积及卵体积均有极显著影响．不同食堕种空中,种群增长率以小球藻组最小,栅藻组最大;个体体积以小球藻组最小,栅藻组和混合藻组问无显著差异．种群增长率(Y,d^-1)与食物浓度(X,×10^6cells·ml^-1)间呈曲线相关,两者间的关系符合方程：y＝-0．0040x^2+0．0409X+0．4471。在所研究的食物浓度范围内,当浓度分别高于或低于6．0×10^cells·m1^-1和3．0×10^6cells·ml^-1时,轮虫个体体积和卵体积均分别呈减小的趋势。     

Life history response and age-specific tolerance to starvation in Brachionus plicatilis O.F. Müller (Rotifera)

To examine the life history response and age-specific tolerance to starvation in the rotifer Brachionus plicatilis O.F. Müller, we carried out two series of individual culture experiments. In the first experiment, rotifers were fed until each of the ages of 1-4 days, and were then starved during the rest of their lifetimes. The control group was fed throughout their lifespans. Rotifers stopped active reproduction just after the onset of food deprivation, and showed shorter subsequent survival times when they were starved at older ages. The finding that the larger the number of offspring produced before food deprivation, the shorter the subsequent lifetime under starvation, appeared to reflect a trade-off with the cost of reproduction. In the second experiment, newborns were starved until each of the ages of 1-5 days, and were fed thereafter. The lifespans of the rotifers starved up to the age of 3 days were not statistically different from those that were not starved. Although the starved rotifers began to reproduce once fed again, their lifetime fecundity decreased significantly from that of the non-starved group. Based on these results, it was suggested that the reproductive suppression caused by starvation would cause rotifers to have a longer lifespan to allow for future reproduction.     

Behaviour, growth and survival of redfish larvae in relation to prey availability

Growth, survival and condition of redfish larvae Sebastes spp., reared in the laboratory (0, 500 1500 and 4500 prey l^-1) were highest in the 1500 prey l^-1 treatment. Significantly lower larval growth and survival in the 4500 prey l^-1 treatment corresponded with lower prey bite: orient ratios in later weeks, suggesting that larvae were unable to forage efficiently at high prey densities. While these prey densities are higher than those reported in the field, naturally co-occurring Atlantic cod Gadus morhua larvae require higher prey densities when reared under similar conditions in the laboratory. These data suggest that prey availability may not be as limiting to redfish as for other commercially important marine species.     

Life table demography and population growth of Brachionus variabilis Hempel, 1896 in relation to Chlorella vulgaris densities

We studied the life history variables and population growth characteristics of Brachionus variabilis, which was recorded for the first time from Mexico. The animals were fed Chlorella, using five concentrations (0.25, 0.5, 1, 2 and 4 × 10⁶ cells ml^–1) at 25 °C. Food density was observed to have significant effect on life expectancy, average lifespan, gross reproductive rate, net reproductive rate, generation time and population growth rate. The average lifespan ranged from 3 to 6 days depending on the food density. The net reproductive rate ranged from 2 to 7 neonates female^–1 d^–1. The rate of population increase per day varied from 0.14 to 0.35. The highest net reproductive rate and average lifespan and life expectancy were recorded at Chlorella concentrations of 1 × 10⁶ and 2 × 10⁶ cells ml^–1.     

Increase of rotifer diversity after sewage diversion in the hypertrophic lagoon,Albufera of Valencia,Spain

The Albufera of Valencia is a large oligohaline hypertrophic lagoon, regulated by sluice gates according to the needs of the surrounding rice field cultivation. It is in a turbid state with permanent cyanobacterial blooms. A slight improvement was detected after diversion in the 1990s of part of the sewage flowing into it. After sewage diversion, we found that: (1) Chlorophyll concentration and rotifer densities decreased; (2) Rotifer proportions declined, due mainly to a relative increase in cladocerans; (3) Rotifer diversity increased. The two dominants of the 1980s, Polyarthra spp. in the colder period and Brachionus angularis in the warmer one, reverted after sewage diversion to a more diverse assemblage reminiscent of the 1970s, with a higher number of dominant species. In the summer of 1998, both Brachionus calyciflorus and its predator Asplanchna brightwelli, dominant in 1973, became abundant again. In 1998, an increase in the number of dominant species was also observed during water renewal periods, some of these species were new or seldomly found before in the lagoon (Proalides tentaculatus-digitus, Trichocerca pusilla at the end of rice culture, Brachionus variabilis at the end of winter flooding). Another change that indicates an improvement of water conditions is a more distinct and longer clear water phase, which occurs in the water renewal period at the end of winter and involves a Daphnia magna peak. The increased importance of this phase, promoted the flourishment of Brachionus variabilis, a facultative Daphnia epibiont never found before in the lake.     

Why do rotifer populations present a typical sigmoid growth curve?

To determine the underlying processes to population growth in the rotifer Brachionus plicatilis, we conducted an experiment using 1.5 ml cultures for 70 days. All individuals were transferred daily to culture media containing algae, and the number of individuals, clutch sizes and number of deaths were counted. The population dynamics showed a typical sigmoid curve. The population density increased exponentially from 10 to 682 individuals during the first 7 days (exponential growth phase), and gradually up to about 1500 individuals during the next 30 days (post-exponential growth phase). The population density then remained at a constant level with small fluctuations during the rest of the experimental period (stationary phase). Mortalities appeared from the post-exponential growth phase and were almost constant at about 2% throughout the experimental period. The clutch size decreased from 5 to 1 during the first 5 days, and afterwards females laid only one egg each. The proportion of non-reproductive females increased from 30% (exponential growth phase) to 80% (post-exponential growth phase) to 90% (stationary phase). These results suggest that the exponential growth phase resulted from the imbalance between a high birth rate and a low death rate, while the stationary phase was maintained by the compensation between low birth and death rates.     

食物种类和浓度对壶状臂尾轮虫实验种群动态的影响

使用浓度为0.3mg/mL的椭圆小球藻、尖细栅藻和两者以1:1(湿重比)组成的混合藻在26±1℃下对壶状臂尾轮虫进行单个体培养研究。结果表明,虽然三类食物对轮虫的胚胎发育时间和平均寿命无显著影响,但投喂小球藻时轮虫的生殖前期明显比投喂栅藻或混合藻时短,投喂小球藻时轮虫的生殖期明显比投喂栅藻时长;轮虫的生殖后期历时以栅藻组最长,混合藻组次之,小球藻组最短,三者间具显著差异。轮虫的繁殖率、产卵量和种群内禀增长率均以小球藻组最高,混合藻组次之,栅藻组最低。由此可见,小球藻是该种轮虫培养的最适饵料。以浓度为0.375、0.75、1.5、3.0和5.0×106cells/mL的椭圆小球藻为食物在23±1℃下对壶状臂尾轮虫进行单个体培养研究发现,0.75×106cells/mL是其生存和繁殖的最低浓度阈值;食物浓度对轮虫的胚胎发育时间和生殖后期历时无显著影响;食物浓度为1.5×106cells/mL和3.0×106cells/mL时,轮虫各主要发育阶段的历时和产卵量间也无显著差异;食物浓度高于3.0×106cells/mL或低于1.5×106cells/mL对轮虫的生殖前期无显著影响,但使轮虫的平均寿命和产卵量显著减小;食物浓度高于3.0×106cells/mL时,轮虫的生殖期显著缩短。轮虫种群的繁殖率、净生殖率和内禀增长率皆以3.0×106cells／mL组最高,1.5×106cells／mL组次之,0.75×106 cells／mL和5.0×106cells／mL组较低。因此,轮虫种群增长的适宜食物浓度范围为1.5-3.0×106cells／mL,最适食物浓度是3.0×106cells／mL。       

Effect of growth hormone and γ-aminobutyric acid on Brachionus plicatilis (Rotifera) reproduction at low food or high ammonia levels

Growth hormone (GH, 0.0025 and 0.025 I.U. ml⁻¹) and γ-aminobutyric acid (GABA, 50 μg ml⁻¹) enhance rotifer population growth in batch cultures. In order to further understand the mechanism of their actions, we conducted experiments culturing isolated females at low food and high free ammonia levels. At an optimum food level of 7×10⁶ Nannochloropsis oculata cells ml⁻¹ or at low free ammonia level of 2.4 μg ml⁻¹, the F₁ offspring of rotifers treated with GH at 0.0025 I.U. ml⁻¹ had significantly higher population growth rate (r) and net reproduction rate (Ro), and shorter generation time than untreated rotifers. At a lower food level of 7×10⁵ cells ml⁻¹ or at high free ammonia level of 3.1 μg ml⁻¹, rotifers treated with GABA at 50 μg ml⁻¹ had significantly higher r and Ro, and shorter generation time. These results indicate that GABA is effective in enhancing rotifer reproduction when rotifers are cultured under stress whereas GH enhances rotifer reproduction when culture conditions are optimal. Significant effects were also observed in F¹ and F² generations which were not treated with hormones. These data may be useful for treating rotifer mass cultures to mitigate the effects of stress caused by high population densities.     

The Cost of Reproduction in the Rotifer Brachionus calyciflorus Pallas

A study of lifespan, fecundity, and reproductive schedules was carried out with the rotifer Brachionus calyciflorus under different food concentrations. At high food densities (6.1 to 12.4 μg · ml⁻¹, dry mass) there was an increase in offspring number but the reproductive investment remained constantly. This was possible because Brachionus calyciflorus invested a smaller amount of energy one egg volume being able to increase offspring number. Although egg volume was reduced, this was compensated by the greater amount of energy available to the neonates. The length of the prereproductive period remained constant throughout this range of food concentrations. Within this range a reduction in lifespan was observed. This, however, was not due to a total higher reproductive investment, but to an increased offspring rate production resulting in a shortening of the reproductive period. The length of the post-reproductive period was not different at all food concentrations.