Population Growth Experiments Research Articles

First-passage-time statistics of growing microbial populations carry an imprint of initial conditions

In exponential population growth, variability in the timing of individual division events and environmental factors (including stochastic inoculation) compound to produce variable growth trajectories. In several stochastic models of exponential growth we show power-law relationships that relate variability in the time required to reach a threshold population size to growth rate and inoculum size. Population-growth experiments in E. coli and S. aureus with inoculum sizes ranging between 1 and 100 are consistent with these relationships. We quantify how noise accumulates over time, finding that it encodes—and can be used to deduce—information about the early growth rate of a population.

Survival of Pseudomonas syringae pv. actinidiae in detached kiwifruit leaves at different environmental conditions.

Pseudomonas syringae pv. actinidiae (Psa) is the causal agent of kiwifruit canker, a serious threat to commercial kiwifruit production worldwide. Studies of the movement path and the survival time of Psa in the host are crucial for integrated management programs. Hence, we used Psa with GFPuv gene (Psa-GFPuv) strain to investigate the movement path of Psa in leaves and branches, and the survival time of Psa in leaves under different environmental conditions. We found that the pathogen Psa spread longitudinally in the branches and leaves rather than transverse path. Additionally, the survival time of bacteria in fallen leaves under different environmental conditions were simulated by the way of Psa infecting the detached kiwifruit leaves. Psa survives the longest, up to 43 days in detached kiwifruit leaves with high humidity (above 80%) at 5 °C, and up to 32 days with low humidity (20%). At 15 °C, the Psa can survive in detached kiwifruit leaves for 20-30 days with increasing humidity. At 25 °C, it can only survive for 3 days with low humidity (20%) and 15 days with high humidity (above 80%). Furthermore, the population growth experiments showed that bacterial growth of Psa was more favorable in detached kiwifruit leaves with above 80% humidity at 5 °C. These results suggest that the survival condition of Psa in detached kiwifruit leaves is significantly affected by environmental conditions, and provide the basis for the control timing and technology of kiwifruit canker.

Rapid adaptation of Brachionus dorcas (Rotifera) to tetracycline antibiotic stress.

Although natural populations can rapidly adapt to selection pressures, the fitness consequences of selection are controversial. In this study, a selection experiment was conducted with replicate populations of Brachionus dorcas that were exposed to two sublethal concentrations (26.8 and 78.3mg/L) of oxytetracycline (OTC), followed by two common garden experiments (population growth and life table experiments). During the 102-day (approximately 36 asexual generations) selection experiment, a markedly increased growth rate but a significantly decreased mictic ratio over time in the populations exposed to OTC when compared to the control populations suggested that the former adapted to the selection pressures and that a trade-off exists between asexual and sexual reproduction. The high and stable population growth rates after 90 days of OTC selection illustrate an example of evolutionary rescue. After 102 days of selection, OTC-selected populations showed higher population growth rates than the control populations when exposed to OTC, indicating significantly increased tolerance. OTC-selected populations showed a lower average growth rate, longer average generation time and life expectancy at hatching, and higher average net reproduction rate and proportion of mictic offspring than the control populations in the absence of OTC, which indicate that OTC selection results in two fitness costs and three fitness gains and that the effect of OTC selection on fitness differs with the measured fitness variables. Both the evolutionary potential of populations under the stress of higher concentrations of OTC and the fitness costs and gains of selection in the absence of OTC indicate that past exposures to pollutants cannot be neglected when evaluating the effects of current stressors on natural populations.

Toxicity of cyanobacterial blooms from the reservoir Valle de Bravo (Mexico): A case study on the rotifer Brachionus calyciflorus

Proliferating cyanobacterial blooms due eutrophication in reservoirs is a major global problem. The production of cyanotoxins often increases with grazing pressure and temperature while the sensitivity of zooplankton to cyanotoxins is directly related to temperature. Here we evaluate the effect of different concentrations of the crude extract of cyanobacteria from Valle de Bravo reservoir during dry (January) and rainy (September) seasons at 20 and 25 °C on the rotifer Brachionus calyciflorus based on acute and chronic toxicity tests. We filtered 20 or 150 l of lake water, depending on the intensity of the bloom, and estimated the density and diversity of the cyanobacteria. The crude extracts, after 5 cycles of freezing, thawing and sonication at 14 MHz, were filtered and the microcystin concentration quantified based on ELISA. The extracts were used to conduct the acute and chronic toxicity tests, all in quadruplicate. Acute toxicity tests were based on 24 h mortality. Chronic toxicity tests (population growth and life table experiments) were conducted at 5 and 10% of the median lethal concentration. The field samples were dominated by Microcystis sp. (January) or Woronichinia naegeliana (September). The microcystin concentration in lake water was 9.57 μg/l and 0.097 μg/l and the median lethal concentration was 5.34 μg microcystin/L and 0.35 μg microcystin/L in January and September, respectively. Survival and reproduction of B. calyciflorus were lower in the presence of the cyanobacteria crude extract, more so at 20° than at 25 °C. Our results highlight the urgency of regular monitoring based on zooplankton assays for reservoirs in tropical and temperate regions, subject to frequent and dominant cyanobacterial blooms, often as a result of climate change.

Maternal care, larviparous and oviparous reproduction of Hypoaspis larvicolus (Acari: Laelapidae) feeding on astigmatid mites.

Hypoaspis larvicolus (Acari: Laelapidae) (first report from Turkey) occurred together with Sancassania polyphyllae (Acari: Acaridae) on the larvae of the scarab beetle, Polyphylla fullo (Coleoptera: Scarabaeidae), that were feeding on the roots of strawberry in Aydin, Turkey. Laboratory studies were conducted to (1) observe whether H. larvicolus feeds and completes its life cycle on the various stages of S. polyphyllae or other astigmatid mites, such as Acarus siro, Carpoglyphus lactis and Tyrophagus putrescentiae (Acaridae), and to determine its population growth when feeding on these prey, and (2) to determine development periods, longevity and fecundity of H. larvicolus feeding on C. lactis. Hypoaspis larvicolus females did not feed on S. polyphyllae, but fed, developed and reproduced when A. siro, C. lactis or T. putrescentiae were provided as prey. Hypoaspis larvicolus is larviparous as well as oviparous. The female lays eggs or gives birth to larvae. If a female gives birth to a larva, it is attached under the female's venter for 1-2days, a phenomenon recorded for the first time in Hypoaspis; in fact, for the first time in mites. The results of the population growth experiments revealed that H. larvicolus feeding on C. lactis produced the highest number of eggs, juveniles and adults. The developmental periods of H. larvicolus feeding on C. lactis at life-cycle path I (larva to adult) and II (egg to adult) were 12.2 ± 0.3 and 15.6 ± 0.6days (females) and 19.5 ± 0.2 and 20.9 ± 0.4days (males), respectively. Longevity of females versus males of H. larvicolus was 120.6 ± 7.2 versus 91.6 ± 13.1days (life cycle I) and 110.0 ± 27.7 versus 118.3 ± 10.9days (life cycle II), respectively.

Demographic and competition studies on Brachionus ibericus and Proales similis in relation to salinity and algal (Nannochloropsis oculata) density

We isolated the rotifers Brachionus ibericus and Proales similis from the sediment of shrimp tanks and studied their individual demographic characters and competition between them at two food levels (0.25 × 106, 1.00 × 106 cells ml−1 of Nannochloropsis oculata at 25 °C) and salinities ranging from 10 to 30‰. Our hypothesis was that growth rates would be higher with increasing food levels and salinities. Observations were taken twice a day for life table studies and daily once for population growth experiments. Using survivorship and fecundity data, we derived various life history variables. Although the average life span (7.6 ± 0.4 days) and gross reproductive rate (33.8 ± 2.9 neonate female−1 day−1) of B. ibericus were higher than those of P. similis (average life span 5.4 ± 0.6 days and gross reproductive rate 13.0 ± 0.6 neonate female−1 day−1), the population growth experiments showed that P. similis had higher r values (0.32 ± 0.005 day−1) than B. ibericus (0.23 ± 0.002 day−1) at 1.0 × 106 cells ml−1 of N. oculata. The rotifer P. similis was more adversely affected due to the presence of B. ibericus than vice versa. The data are important for developing techniques for a large-scale culture of these rotifers as food in aquaculture.

Effect of the allelochemicals from the macrophyte Egeria densa on the competitive interactions of pelagic and littoral cladocerans

ABSTRACTMacrophytes release allelochemicals, which affect pelagic cladocerans such as Daphnia. Using population growth experiments, we analysed the effects of allelochemicals from the macrophyte Egeria densa on the interaction between Daphnia mendotae and three littoral cladocerans (Diaphanosoma birgei, Macrothrix triserialis and Simocephalus mixtus). We found that allelochemicals from E. densa increased the abundance of all the tested cladocerans in spite of the presence of a competitor. This effect was stronger (nearly three to four times higher than in controls) for D. birgei and M. triserialis in the absence of D. mendotae. Independent of the presence of allelochemicals, S. mixtus, but not D. birgei and M. triserialis, reduced the abundance of D. mendotae as compared to controls. The rate of population increase (r) per day was significantly elevated due to the presence of Egeria’s allelochemicals (from 0.07–0.16 d−1 without allelochemicals against 0.12–0.24 d−1 with allelochemicals). In our competition experiments, the rates of population increase of the cladocerans were lower than those in single-species cultures when cultured in the absence of allelochemicals. However, in the presence of allelochemicals this trend was not consistent. Thus, the positive effects of Egeria’s allelochemicals over the cladoceran populations may enhance the grazing pressure on phytoplankton.

Demography and feeding behavior of Stenostomum leucops (Dugés, 1828)

<p>Freshwater turbellarians, despite their mainly benthic habits, interact with pelagic communities of rotifers and cladocerans. However, very little is known about their demographic characteristics, food preference and functional response. To fill that gap we studied one of the most widely spread species, <em>Stenostomum leucops</em>. We conducted population growth experiments using abundant food (several rotifer and cladocerans species). To evaluate possible impact of <em>S. leucops</em> on planktonic communities, we conducted prey preference experiments at two temperatures: 18 and 23°C. The number of rotifers and cladocerans consumed was calculated by the difference between the initial and final density. We found that diets supplemented with fresh algae achieved higher <em>S. leucops</em> densities than those with detritus supplements in their diets. In the case of animal diets, <em>Euchlanis dilatata</em> allowed <em>S. leucops</em> reach higher densities than the other zooplankton species; <em>E. dilatata</em> was positively selected for in the selectivity study at both 18 and 23°C. <em>Stenostomum leucops</em> showed a type II functional response on rotifers and the cladoceran <em>Alona glabra</em>. Our results suggest that <em>S. leucops</em> select their prey according to their vulnerability using different mechanisms, which optimize their food intake.</p>

Direct and indirect effects of invertebrate predators on population level responses of the rotifer Brachionus havanaensis (Rotifera)

Rotifers, in nature, are consumed by several vertebrates and invertebrates. In response to predation, they show a certain degree of plasticity in their life history traits. In this regard, there is an ample database of information on the effect of Asplanchna on brachionids; much less is known on the effect of other common freshwater predators. We studied the chemical effects of four invertebrate predators, Stenostomum leucops (Turbellaria), Asplanchna girodi (Rotifera), Eosphora najas (Rotifera) and Mesocyclops pehpeiensis (Copepoda) on the demography of Brachionus havanaensis. Population growth and life table demography experiments were conducted in 50 mL vessels with 25 mL of moderately hard water and 0.5 × 106 cells mL−1 of Chlorella vulgaris as food. Two predators of each phylum were introduced into small meshes with 50 µm mesh and fed 15 individuals of another rotifer species Plationus patulus. Each experiment was set up with two controls, with and without the prey (P. patulus) in meshes, as in all the other treatments. We started the experiments with 15 individuals of B. havanaensis in the population growth experiments and 15 neonates (<12 h old) in the life table studies. The population growth studies were terminated after 20 days and the life table studies after the last individual of each cohort died. We also tested the feeding rates of A. girodi, S. leucops and M. pehpeiensis on B. havanaensis by observing the prey consumed at different time intervals from 15 to 90 min. The presence of P. patulus alone had an impact on the reproductive but not on the survivorship variables of Brachionus as compared to the controls. We found that among the predators, S. leucops had the least impact on the population dynamics and life history variables. The survivorship ranged from 16 to 18 days with no significant impact of infochemicals from any of the test predators. The population growth rate of B. havanaensis ranged from 0.2 to 0.34 day−1 and was highest in the presence of M. pehpeiensis and A. girodi. We also observed maximal differences in the life history strategy of B. havanaensis, as compared to the controls, in response to the most voracious predator, A. girodi.

Competitive outcome between the rotiferBrachionus calyciflorusand the cladoceranMoina macrocopadepends on algal density but not temperature

The rotifer Brachionus calyciflorus often occurs simultaneously with the cladoceran Moina macro- copa in tropical water bodies. We studied the effects of temperature (20, 25 and 30 xC) and algal density (0.5, 1.0 and 3.0r 10 6 cells.mL x1 Scenedesmus obliquus) on the outcome of competition and duration of coex- istence between B. calyciflorus and M. macrocopa using population growth experiments. The results showed that regardless of temperature, M. macrocopa outcompeted B. calyciflorus at the lowest algal density (0.5r 10 6 cells.mL x1 ), whereas B. calyciflorus outcompeted M. macrocopa at the higher algal densities (1.0 and 3.0r 10 6 cells.mL x1 ). The duration of coexistence shortened with increasing temperature when M. macrocopa outcompeted B. calyciflorus, and those with increasing temperature and algal density when B. calyciflorus outcompeted M. macrocopa. Our results suggested that the competitive outcome between the rotifer B. calyciflorus and the cladoceran M. macrocopa was dependent on algal density but not temperature, the duration of coexistence between them was dependent on algal density and temperature, and the exploitative competition of M. macrocopa over B. calyciflorus may be more important than the interference competition.

密度信号对两种臂尾轮虫种群增长的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 密度信号对两种臂尾轮虫种群增长的影响 DOI: 10.5846/stxb201303010331 作者: 作者单位: 生物多样性与生态工程教育部重点实验室,生物多样性与生态工程教育部重点实验室 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金资助项目(30970349) Effects of crowding signals on population growth of two Brachionus rotifer species Author: Affiliation: Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering,College of Life Sciences,Beijing Normal University,Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering,College of Life Sciences,Beijing Normal University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:为了更好的了解化学干扰对轮虫种间竞争的影响,分别用萼花臂尾轮虫(Brachionus calyciflorus)和角突臂尾轮虫(Brachionus angularis)研究了同种和异种种群密度信号对种群增长的影响。结果表明,密度信号对轮虫种群增长具有一定的影响。萼花臂尾轮虫的密度信号显著降低了本种和角突臂尾轮虫种群的环境容纳量K并提高了它们的种群增长率r(P < 0.05),角突臂尾轮虫的密度信号对本种和萼花臂尾轮虫的种群增长都没有显著影响(P > 0.05)。萼花臂尾轮虫种群增长受同种密度信号抑制十分显著,环境容纳量K降低了50%以上,可能主要受自毒作用的调节,但同种密度信号对高有性生殖的萼花臂尾轮虫种群的混交率影响不明显。角突臂尾轮虫几乎不受自身密度信号的影响。因此,在两种臂尾轮虫的竞争中,由于自身密度信号的抑制作用,萼花臂尾轮虫将更快被角突臂尾轮虫排除。据此可推测密度信号对轮虫的种间竞争具有重要影响。 Abstract:Chemical signal is one of the primary channels of sensory input for zooplankton. Chemicals released by zooplankton individuals can affect their behavior, including migration, feeding, predator defense, growth, reproduction, and so on. Rotifers are adequate model animals for chemical ecology studies in aquatic system, with a rich literature on inducible defense and sexual production. However, studies associated with the effects of chemical signals on population competition have not been reported. Brachionus calyciflorus and Brachionus angularis are both common freshwater rotifer species. B. angularis, who has a lower threshold food concentration because of its smaller body size, would out-compete large body sized B. calyciflorus when the resource is limited. The present work examined the effect of crowding signals on population growth of B. calyciflorus and B. angularis. Culture medium conditioned by rotifers was used as the crowding signals and two crowding levels were set for each rotifer species. Population growth experiments of the two rotifers were carried out at an initial density of 1 ind./mL in 5 ml and 20 ml medium for B. angularis and B. calyciflorus, respectively. Regardless of the treatments, B. angularis reached its maximum population density at 100 ind./mL after 23 days, while it took B. calyciflorus only 3 to 8 days to reach its maximum population density, which was far lower than that of B. angularis. Both B. calyciflorus and B. angularis showed a higher population growth rate (r) and lower maximum population density (K) under the crowding signals of B. calyciflorus. However, neither B. calyciflorus population nor B. angularis population was significantly affected by the crowding signals of B. angularis. The maximum population density of B. calyciflorus was inhibited by its own crowding signal (from 9.124 ind./mL to 4.564 and 3.448 ind./mL under low and high crowding signals, respectively), leading a 50% decreasing of the carrying capacity K of the population. That was probably because of the inhibitory effects of autotoxins. Meanwhile, mixis ratios of B. calyciflorus populations fluctuated between 40% and 80%, and showed no significant differences among groups, because B. calyciflorus was sensitive to population density and mixis ratios were high at all these population densities. Because population growth of B. angularis was obviously affected by its own crowding signals, growing to a higher population density, and its threshold food concentration was lower, as a result, exclusion of B. calyciflorus by B. angularis may become quicker during the competition process. These results indicated that crowding signals may play an important role in the competition outcome between different rotifer species. 参考文献 相似文献 引证文献

Effect of food density of Pseudokirchneriella subcapitata and Chlorella vulgaris on the population growth rates of four brachionid rotifers

We conducted population growth experiments for 2–4 weeks using 4 species of brachionid rotifers (Brachionus angularis, B. havanaensis, B. rubens, and Plationus patulus), all isolated from Lake Xochimilco, Mexico City, Mexico. The test rotifers were fed 2 species of green algae (Chlorella vulgaris and Pseudokirchneriella subcapitata) at 3 food concentrations (2.9, 5.8, and 11.6 μg dry weight mL−1) for 24 h. Regardless of algal species, the population density of all 4 rotifer species increased with increasing food density; however, the magnitude of growth rate response of the rotifers differed. At comparable food levels, B. havanaensis had far greater abundances in 3 weeks than the other 3 rotifer species. At a concentration of 2.9 μg mL−1 of C. vulgaris, density of B. angularis was about 50 ind. mL−1, but when the food was enhanced to 11.6 μg mL−1, its population was 3 times higher; similar values were obtained when B. angularis was fed P. subcapitata. In comparison, B. havanaensis fed P. subcapitata at the highest food level had population abundances of about 900 ind. mL−1; at the same food level, when fed C. vulgaris, the density was much lower (250 ind. mL−1). Population abundances of B. rubens fed P. subcapitata were, in general, lower than those fed C. vulgaris. This was also the trend for rotifer P. patulus, which had lower abundances when fed P. subcapitata than when fed C. vulgaris. Regardless of food type and concentration, the rate of population increase per day (r) of the tested rotifer species varied from 0.16 to 0.61 d−1.

Biochemical nutrient requirements of the rotifer Brachionus calyciflorus: co‐limitation by sterols and amino acids

Summary It has been proposed that growth and reproduction of animals is frequently limited by multiple nutrients simultaneously. To improve our understanding of the consequences of multiple nutrient limitations (i.e. co‐limitation) for the performance of animals, we conducted standardized population growth experiments using an important aquatic consumer, the rotifer Brachionus calyciflorus. We compared nutrient profiles (sterols, fatty acids and amino acids) of rotifers and their diets to reveal consumer–diet imbalances and thus potentially limiting nutrients. In concomitant growth experiments, we directly supplemented potentially limiting substances (sterols, fatty acids, amino acids) to a nutrient‐deficient diet, the cyanobacterium Synechococcus elongatus, and recorded population growth rates. The results from the supplementation experiments corroborated the nutrient limitations predicted by assessing consumer–diet imbalances, but provided more detailed information on co‐limiting nutrients. While the fatty acid deficiency of the cyanobacterium appeared to be of minor importance, the addition of both cholesterol and certain amino acids (leucine and isoleucine) improved population growth rates of rotifers, indicating a simultaneous limitation by sterols and amino acids. Our results add to growing evidence that consumers frequently face multiple nutrient limitations and suggest that the concept of co‐limitation has to be considered in studies assessing nutrient‐limited growth responses of consumers.

Morphometric and demographic responses of brachionid prey (Brachionus calyciflorus Pallas and Plationus macracanthus (Daday)) in the presence of different densities of the predator Asplanchna brightwellii (Rotifera: Asplanchnidae)

We quantified the indirect effects of different densities (1, 4, 16 individuals per jar) of the predator Asplanchna brightwellii on the morphometry and demography of their prey Brachionus calyciflorus and Plationus macracanthus. Population growth and life table demography experiments were separately conducted for each of the two prey rotifer species while keeping A. brightwellii in indirect contact. The brachionids were fed the green alga Chlorella vulgaris at a density of 1 × 106 cells ml−1. As compared to those cultured in the absence of the predator, in the presence of A. brightwellii the postero-lateral spines of P. macracanthus increased by about 15 μm, while in B. calyciflorus the increase was more (>80 μm). For both the brachionid species, the peak population density significantly decreased in the presence of A. brightwellii. The reproductive variables viz., net reproductive rate, generation time, and the rate of population increase of B. calyciflorus and P. macracanthus were negatively affected in the presence of kairomones from A. brightwellii.

The effects of temperature and salinity on population growth and egg hatching success of the tropical calanoid copepod, Acartia sinjiensis

The tropical calanoid copepod, Acartia sinjiensis, has good potential for mass culture as a live feed for tropical reef fish larvae. This study was carried out to investigate the effects of temperature and salinity on population growth and egg hatching rates of A. sinjiensis. At a salinity of 30±1 psu, temperature effects on population growth of A. sinjiensis was determined after 8 day culture at eight temperatures of 10, 15, 20, 25, 28, 30, 34 and 38 °C. Adopting similar experimental protocols, effects of salinity on population growth of A. sinjiensis were investigated under nine salinities of 10, 15, 20, 25, 30, 35, 40, 45 and 50 psu, with culture temperature set at 30±1 °C. In addition to population growth experiments, egg hatching rates (%) after 48 h of incubation at various temperature and salinity conditions were also examined. For all experiments, five replicates were set up for each treatment. A. sinjiensis were fed an identical binary algal diet of Tetraselmis chuii and the Tahitian strain of Isochrysis sp. (T-ISO), photoperiod was maintained at 12 L:12D.The results showed that temperature had significant effects on both population growth and egg hatching rates. Population growth at 30 °C (with an initial number of 10 adults/replicate, final mean population reached 677.2±136.6) was significantly higher (p<0.001) than the other temperature treatments except that of 25 °C and 28 °C treatments. The egg hatching rate did not differ significantly (p<0.05) when incubated at temperature 25, 30 and 34 °C although the highest hatching rate was recorded at 34 °C (96.3±2.2%). Salinity also had significant effects on both population growth and egg hatching rates. The highest population growth (final mean population: 669.0±111.3) and egg hatching rate (98.0±1.4%) both occurred at salinity of 30 psu. Population growth at 30 psu was significantly higher (p<0.001) than all other treatments except that of 25 and 35 psu, while there was no significant difference (p<0.05) between egg hatch rates at all salinity levels tested.The results of this study suggest that for maximum population growth and egg hatching success, A. sinjiensis should be cultured at 30 °C with a salinity of 30 psu.

Competition between the Rotifer Brachionus calyciflorus and the Cladoceran Moina macrocopa in Relation to Algal Food Concentration and Initial Rotifer Population Density

ABSTRACT The rotifer Brachionus calyciflorus occurs simultaneously with the cladoceran Moina macrocopa in tropical water bodies, and mass cultures of M. macrocopa are sometimes contaminated with B. calyciflorus. The effects of algal food concentration (Chlorella vulgaris) and initial rotifer population density on the competition between B. calyciflorus and M. macrocopa were tested using population growth experiments, and the influence of adult M. macrocopa on the instantaneous reproductive rates, egg ratios, and survivorship of B. calyciflorus females was investigated in short-term experiments. M. macrocopa rapidly excluded B. calyciflorus from mixed-species cultures but was itself unaffected by the presence of the rotifers, regardless of the algal food concentration or initial rotifer population density. Adult M. macrocopa imposed significant negative effects on the instantaneous reproductive rates, egg ratios, and survivorship of newborn individuals of B. calyciflorus. The magnitude of effects imposed by M. macrocopa increased with decreased food level and increasing density of adult M. macrocopa.

Life History Characteristics of Asplanchnopus multiceps (Rotifera) Fed Rotifer and Cladoceran Prey

Members of rotifer family Asplanchnidae are important invertebrate predators in freshwater communities. Although a considerable amount of information exists on species of Asplanchna, relatively less is known about Asplanchnopus. We isolated Asplanchnopus multiceps from the littoral of a small river in the State of Hidalgo in Central Mexico and separated a clone in our cultures. The gut content analysis of some animals collected from the field revealed the presence of cladocerans and rotifers, and therefore we cultured A. multiceps on a food mixture comprising littoral rotifers and cladocerans. We conducted population growth experiments of A. multiceps using six prey types (cladocerans: Macrothrix triserialis, Alona rectangula and Pleuroxus aduncus; rotifers, Brachionus patulus, B. macracanthus and B. urceolaris). The prey species (A. rectangula and B. patulus) on which the highest growth rates were observed were used to test the life-table demographic patterns in A. multiceps. All experiments were conducted in 50 ml containers with 25 ml of the medium and at three food levels (0.5, 1.0 and 2.0 ind. ml−1 for the cladocerans, and 2.0, 4.0 and 8.0 ind. ml−1 for the rotifers) with four replicates at each treatment. The spines of M. triserialis and B. macracanthus were apparently effective deterrents against Asplanchnopus predation since both these diets resulted in low, and sometimes negative, growth rates of the predator. The average lifespan and net reproductive rate of A. multiceps ranged from 3.8 to 8.4 days and 2.6 to 12.2 ind. female−1, respectively, on A. rectangula; and from 5.0 to 9.4 days and 1.6–18.4 ind. female−1, respectively, on B. patulus. The rate of population increase of A. multiceps ranged from 0.1 to 0.8 d−1, depending on the prey type and density. The role of A. multiceps in structuring littoral rotifer and cladoceran communities is discussed.

Detrimental and neutral effects of wild barley–Neotyphodium fungal endophyte associations on insect survival

Abstract Neotyphodium (Clavicipitaceae: Balansieae) fungal endophyte infection does not always confer temperate grass resistance to insect herbivores, although reports indicate that over 40 species are adversely affected by its infection. Laboratory and glasshouse experiments were conducted to improve our knowledge of the anti‐insect properties of Neotyphodium‐infected (E+) non‐commercial grasses, and E+ wild barley (Hordeum) specifically. Neotyphodium infection of four plant inventory (PI) lines of wild barley conferred resistance to Mayetiola destructor (Say) (Diptera: Cecidomyiidae), whereas none of the E+ wild barley accessions reduced the survival of Rhopalosiphum padi (L.) (Homoptera: Aphididae). Metopolophium dirhodum (Walker) (Homoptera: Aphididae) densities were significantly lower on the E+ clones of Hordeum brevisubulatum ssp. violaceum (Boissier and Hohenacker) (PI 440420), compared with densities on endophyte‐free (E–) plants of this species in population growth experiments. Neotyphodium infection of three Hordeum bogdanii (Wilensky) PI lines did not confer resistance to M. dirhodum; however, one of these E+ lines (PI 314696) was resistant to this aphid in a second population growth experiment. Our results provide additional evidence that the outcome of a grass–endophyte–insect interaction is influenced by the host grass species or genotype, Neotyphodium species or genotype, and the insect species involved. They also reinforce this phenomenon for non‐commercial grass–endophyte–insect interactions and underscore the potential role of endophytes in mediating wild barley–insect interactions and their potential to act as defensive mutualists.

Respiratory electron transport system (ETS) activity as an estimator of the thermal tolerance of two Daphnia hybrids

The influence of temperature on the activity of the respiratory electron transport system (ETS) was measured in one clone ofDaphnia hyalina x galeata and one ofDaphnia cucullata x galeata, isolated from Lake Bled (Slovenia). The ETS activity of ovigerous females acclimated to 7, 20 and 25° C, was measured at 5, 10, 15, 20, 25 and 30°C. Population growth experiments showed that D. cucullata × galeata grew better at high rather than low temperatures. Daphnia hyalina x galeata, however, grew more successful at low temperatures than did D. cucullata x galeata. The highest Q 10 of ETS activity of D. cucullata x galeata at the lowest temperature range of 5-15°C indicated the absence of enzymes that could function sufficiently well at low temperatures. The ETS activity of the warm-acclimated hybrid D. hyalina x galeata reached a maximum at an incubation temperature of 20°C, while D. cucullata x galeata had maximal ETS activity at 25°C. Thus D. cucullata x galeata has a more efficient enzyme system than D. hyalina x galeata at the higher temperature. The higher Arrhenius activation energy (E a ) for D. cucullata x galeata than for D. hyalina x galeata indicates that enzymes from D. cucullata x galeata are more temperature sensitive than those from D. hyalina x galeata. In conclusion, the ETS of D. cucullata x galeata is adapted to a higher temperature and to narrower temperature fluctuations than that of D. hyalina x galeata.

The efficacy of Scenedesmus morphology as a defense mechanism against grazing by selected species of rotifers and cladocerans

Phytoplankton often develop various defense mechanisms in response to zooplankton grazing, such as spines and colonies. While it is now known that increased spine length and cells in a colony of members of the genus Scenedesmus, when zooplankton grazing is intense, helps in reducing zooplankton filtering rates, the effect of these defense mechanisms at the population level has been observed in few studies. Here we present data on the growth rates of four zooplankton species, Brachionus calyciflorus, B. patulus, Ceriodaphnia dubia and Daphnia pulex at two food levels using two species of colony-forming Scenedesmus spp.: S. acutus (cell length = 18.2 ± 0.4 µm; width = 4.2 ± 0.1 µm; average colony length = 90 µm; width: 21 µm) and S. quadricauda (cell length: 21 ± 0.5 width 7.5 ± 0.3 µm; average colony length: 84 µm; width: 30 µm). Whereas S. acutus had no spines, S. quadricauda had spines of 6–10 µm. Population growth experiments of the test rotifers and cladocerans were conducted in 100 ml containers with 50 ml of the medium with test algae. Algae concentrations used were: 13 and 52 mg dw l−1 of each of the two algal species offered in colonial forms. We used an initial inoculation zooplankter density of 1 ind. ml−1 for either of the rotifer species and 0.2 ind. ml−1 for either of the cladoceran species. In all, we had 64 test containers (4 test species of zooplankton × 2 test species of algae × 2 algal densities × 4 replicates). We found a significant effect of algal size on the growth rates of all the four tested species of zooplankton. The population growth rates of zooplankton ranged from −0.58 to 0.66 and were significantly higher on diet of S. acutus than of S. quadricauda. Thus, our study confirms that the larger colony size and the formation of spines in S. quadricauda were effective defenses against grazing by both rotifers and smaller sized cladoceran Ceriodaphnia dubia but that larger-bodied Daphnia pulex could exploit both the algal populations equally.