Demography and population growth of Asplanchna girodi (Rotifera) as a function of prey (Anuraeopsis fissa) density

Laboratory studies on population growth and life table demography of Asplanchna girodi were conducted at 25±1 °c using Anuraeopsis fissa as prey at four (250, 500, 1000 and 2000 ind ml^–1) densities. A prey density of 100 ind ml^–1 per predator per day did not support A. girodi, while at the highest prey concentration, A. girodi reached a peak of 115±7 ind ml^–1. The age distribution of A. girodi indicated that non-adults constituted about 2/3rd of the population at all prey concentrations. A decrease in prey availability resulted in increased mortality of non-adults. At the highest prey density, the rate of population increase (r) was 1.51 d^–1. The significance of estimating mortality in population growth studies is discussed.Life table demography of A. girodi was also studied using the same prey at the same concentrations. None of the survivorship parameters (e.g. mean lifespan and mean survivorship) showed a significant relation to prey density. Net reproductive rate and generation time (but not rate of population increase) were affected by prey abundance.     

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.     

Combined Effects of Food Concentration and Temperature on Competition Among Four Species of <Emphasis Type="Italic">Brachionus </Emphasis>(Rotifera)

We evaluated the effect of algal food density (1.5 × 10⁶, 3.0 × 10⁶ and 4.5 × 10⁶ cells ml⁻¹ of Chlorella) and temperature (22° and 28 °C) on competition among the rotifers Brachionus calyciflorus, Brachionus havanaensis, Brachionus patulus and Brachionus rubens, based on population growth experiments for 24 days. The growth experiments were conducted seperately for each individual rotifer species (i.e., controls), and in mixtures of all four species in equal initial proportions (i.e., under competition). The population growth of B. calyciflorus, B. havanaensis, B. patulus and B. rubens grown separately at two temperatures and at three algal food densities showed typical patterns of lag, exponential and retardation phases in the controls. This pattern differed considerably under competition. In general, we observed that in all of the test species, the highest growth rates were observed at higher food levels and in the absence of congenerics. At 22 °C, under the lowest food level, the differences in the population abundances of B. havanaensis, B. patulus and B. rubens grown alone and in the presence of competition were large. However, these differences reduced as food density was increased from 0.5 × 10⁶ to 4.5 × 10⁶ cells ml⁻¹. At 28 °C and at the lowest food level, all of the other rotifer species eliminated B. havanaensis in mixed cultures. Each brachionid species had a higher rate when grown alone than when cultured with other species. The highest r (mean ± standard error: 0.54 ± 0.01 day⁻¹) was recorded for B. havanaensis at 28 °C under 4.5 × 10⁶ cells ml⁻¹ of algal food density. At 28 °C at low algal food density, the presence of competitors resulted in negative population growth rates for three of the four rotifer species tested.     

High density culture of the freshwater rotifer,Brachionus calyciflorus

The freshwater rotifer, Brachionus calyciflorus is one of the live food organisms used for the mass production of larval fish. In this study possibility of obtaining high density cultures of the freshwater rotifer B. calyciflorus were investigated. The two culture systems used differed in their air and dissolved oxygen supplies using three temperatures in each case: 24, 28 and 32 °C. Rotifers were batch-cultured using 5 l-vessels and fed with the freshwater Chlorella. The growth rate of rotifers significantly increased with an increase in temperature. The maximum density of the rotifers with air-supply at 24 °C, 6500 ind. ml^–1, was significantly lower than those cultured at 28 and 32 °C, i.e. 8600 and 8100 ind. ml^–1, respectively. Dissolved oxygen levels decreased with time and ranged from 0.8 to 1.4 mg l^–1 when the density of freshwater rotifer was the highest at each temperature. The highest density (19200 ind. ml^–1) of freshwater rotifer was obtained in cultures with a supply of oxygen at 28 °C. Densities of 13500 and 17200 ind. ml^–1 were found at 24 and 32 °C, respectively. Levels of NH₃-N increased with time and a dramatic increase of NH₃-N was observed at high temperatures. Levels of NH₃-N at 24, 28 and 32 °C were 13.2, 18.5 and 24.5 mg l^–1, respectively. These levels coincided with the highest rotifer density at each of the three temperatures. When rotifers were cultured with an oxygen-supply and pH was adjusted to 7, the maximum density of rotifer reached 33500 ind. ml^–1 at 32 °C . These results suggested that high density culture of freshwater rotifer, B. calyciflorus could be achieved under optimal conditions with DO value of exceeding 5 mg l^–1 and NH₃-N values of lower than 12.0 mg l^–1.     

Brachionus calyciflorus Pallas as agent for the removal of E. coli in sewage ponds

Brachionus calyciflorus (Pallas) is a common brachionid in sewage oxidation ponds. The uptake and assimilation of E. coli was optimal at concentrations of 2.7–6.9 × 10⁸ cells ml^–1 while assimilation coefficient per body weight of B. calyciflorus was found to be 10% · Ind.^–1 d^–1. More than two eggs per individual were produced during 24 hours when brachionids were fed with a mixutre of E. coli (10⁹ cells · ml^–1) and Chlorella spp. (10⁶ cells · ml^–1). The nutritional value of the mixture of E. coli and Chlorella spp. was found to be higher than that of bacteria alone.     

Studies on functional response and prey selection using zooplankton in the anostracan Chirocephalus diaphanus Prevost, 1803

Freshwater cladocerans and rotifers were used as prey to study functional response and prey selection by adult females of Chirocephalus diaphanus under laboratory conditions. For functional response studies, we offered three rotifer species (Brachionus calyciflorus, B. patulus and Euchlanis dilatata) and three cladoceran species (Alona rectangula, Ceriodaphnia dubia and Moina macrocopa) at various densities ranging from 0.5 to 16 ind. ml^–1. We found increased zooplankton consumption with increasing prey density but beyond 4 ind ml^–1 cladocerans and 8 ind. ml^–1 rotifers, the number of animals eaten plateaued. In general, C. diaphanus consumed fewer large prey (cladocerans) and many more smaller zooplankton (rotifers). For prey selection experiments, we used B. calyciflrous, B. patulus, C. dubia and M. macrocopa, offered at the ratio of two rotifers: one cladoceran and at three prey densities (total zooplankton numbers: 3, 6 and 12 ind. ml^–1). Prey selectivity patterns followed the functional response trends. In general, regardless of prey types, with an increase in the available zooplankton, there was an increase in the number of prey consumed. At any given prey density, C. diaphanus consumed higher numbers of rotifers than cladocerans. Among the prey offered, B. patulus and M. macrocopa were positively selected. Results are discussed in light of possible control of zooplankton by anostracans in temporary ponds.     

Utilization of cyanobacteria by Brachionus calyciflorus: Anabaena flos-aquae (NRC-44-1) as a sole or complementary food source

The rotifer Brachionus calyciflorus can utilize the cyanobacterium Anabaena flos-aquae as either a sole or supplementary food source in laboratory culture. Positive population growth rates accompany food densities of 10 or 100 µg dry weight ml^–1, but slightly negative rates are found at a lower density (1.0 µg ml^–1). These results are consistent for rotifers feeding on two strains of A. flos-aquae, UTEX-1444 and NRC-44-1, with slightly enhanced survivorship and reproduction with the latter food. A 1:1 mixture (by dry weight) of Euglena gracilis and A. flos-aquae (NRC-44-1) produces survivorship comparable to that of control rotifer cohorts fed E. gracilis alone, but elicits significantly greater fecundity and population growth rates than found with the control food suspension at the same biomass density.     

Brachionus plicatilis tolerance to low oxygen concentrations

Tolerance to low oxygen concentrations is expected in Brachionus plicatilis, a rotifer adapted to live in saline warm waters. The population dynamics of a clone of this species, isolated from an endorreic saline lake, was studied under controlled laboratory conditions. Although their growth and metabolism is extremely reduced, B. plicatilis populations are able to maintain relatively high-density populations (a mean of 35 ind ml^–1) in oxygen concentrations below 1 mg 1^–1, for more than one month. Major features of population growth related to oxygen are discussed.     

Population growth of Asplanchna sieboldi fed two Brachionus spp. (Rotifera) raised on green alga and baker's yeast

We conducted population growth experiments of A. sieboldi using Brachionus calyciflorus and Brachionus patulus as prey. The prey rotifers were mass cultured separately on Chlorella vulgaris, Saccharomyces cerevisiae or on their mixture. Data on population growth of A. sieboldi showed prey type and food density-related differences. At any given prey concentration, both B. calyciflorus and B. patulus raised on a mixture of alga and yeast, resulted in higher abundance of the predator than those raised solely on alga or yeast. The rate of population increase per day (r) of A. sieboldi increased with increasing prey density for both prey species. However, predators grown on B. patulus showed higher r values compared to those grown on B. calyciflorus.     

Ecological problems of Lake Ladoga: causes and solutions

We studied the outcome of competition between a large (Brachionus calyciflorus) and a small (Anuraeopsis fissa) rotifer species at five algal (Scenedesmus acutus) concentrations (0.5 × 10⁶ to 40.5 × 10⁶ cells ml^–1) and with varying initial densities in mixed populations (100 to 0% of B. calcyciflorus or A. fissa), the combined initial biomass being 0.2 µg ml^–1 in all test jars. Experiments were conducted at 28 ± 1 °C.Regardless of food concentration, B. calcyciflorus showed a greater increase in biomass than A. fissa, peak densities (mean ± standard error) at the lowest food concentration in the controls being 1.34 ± 0.31 µg dry weight ml^–1 and 0.82 ± 0.08 dry weight ml^–1, respectively. At the lower food concentrations, A. fissa displaced B. calyciflorus and vice versa at the higher food concentrations. At the intermediate food concentrations of 4.5 × 10⁶ cells ml^–1, B. calyciflorus outcompeted A. fissa only if its initial population density was three times higher. The rates of population growth in controls varied from 0.792 ± 0.06 d^–1 to 1.492 ± 0.13 d^–1 for B. calyciflorus and 0.445 ± 0.04 to 0.885 ± 0.01 for A. fissa depending on food level. When both species were introduced together, low food levels favoured higher abundance of A. fissa than B. calyciflorus, suggesting, in nature, it is likely that small Anuraeopsis colonize oligotrophic water bodies more successfully than larger Brachionus. The results also suggest that the outcome of competition depends not only on the size of the competing species and food availability but also on their colonizing density.     

Effect of the cyclopoid copepod Mesocyclops thermocyclopoides on the interactions between the predatory rotifer Asplanchna intermedia and its prey Brachionus calyciflorus and B. angularis

In many shallow, eutrophic subtropical ponds, brachionid rotifers are common prey of the predatory copepod Mesocyclops thermocyclopoides. The predatory rotifer Asplanchna intermedia, which is itself a potential prey of the copepod, also feeds preferentially on brachionids. We studied in the laboratory the population dynamics of two mutually competing prey species, Brachionus angularis and B. calyciflorus, in the presence of the two predators A. intermedia and M. thermocyclopoides. The experimental design included separate population dynamics studies with one prey–one predator, two prey–one predator, one prey–two predator, and two prey–two predator systems. These combinations were compared with controls, in which both the prey species (B. angularis and B. calyciflorus) were grown separately and in combination with each other. In the absence of any predator, B. angularis generally eliminated the larger B. calyciflorus. Selective predation by the copepod allowed B. calyciflorus to persist longer in competition with B. angularis. Feeding by M. thermocyclopoides on A. intermedia reduced the predation pressure on B. calyciflorus. However, given enough time, the cyclopoid copepod was able to eliminate both the brachionids as well as the predatory Asplanchna.     

A study of the freshwater rotifer Brachionus calyciflorus in Pekanbaru, Riau, Indonesia

Brachionus calyciflorus is one of freshwaterrotifers found in fish ponds in Pekanbaru, Riau,Indonesia. Its density varied depending on that ofphytoplankton. Maximum of 2.5 ×10³ ind. l^–1 was found in ponds fertilised with animal wastes.The fecundity and population growth of B.calyciflorus was studied. Results indicated that the fecundity of amictic females was higher thanthat of unfertilized mictic females. During their life span, amictic and mictic females produced29.7 and 12.5 eggs per female, respectively. In the mass culture of 500 ml media, the highestdensity of female and male rotifers was 975.8 and 9.6 ind. ml^–1, respectively. During 8 days cultureperiod, they also produced eggs as many as 124.2 ml^–1.Human and animal wastespromoted the growth of phytoplankton as food forrotifers in the pond. A laboratory study confirms this. The best growth of B. calyciflorus at a density of 109 ind. ml^–1 was found at 0.5 g l^–1 of humanexcreta. A high density of B. calyciflorus (542ind. ml^–1) was also found in semi-continuous culturewith chicken excreta.     

Competition between Brachionus calyciflorus Pallas and Brachionus patulus (Müller) (Rotifera) in relation to algal food concentration and initial population density

Competitive laboratory experiments between Brachionus calyciflorus and B. patulus were conducted at low (1×10⁶ cells ml^–1) and high (3×10⁶ cells ml^–1) densities of Chlorella vulgaris and four initial inoculation densities (numerically, 100% B. calyciflorus; 75% B. calyciflorus + 25% B. patulus; 50% each of the two species; 25% B. calyciflorus + 75% B. patulus and 100% B. patulus). Population densities were enumerated and the medium was changed daily for 20 days. B. patulus was a superior competitor in low food density regardless of inoculation density. At high food density, B. calyciflorus showed higher population growth in the first week but thereafter was outcompeted by B. patulus regardless of initial density. When grown alone, B. calyciflorus reached peak abundances (mean ± standard error) of 31±3 and 81±7 individuals ml^–1 at low and high food densities, respectively. The corresponding values for B. patulus were 130±2 and 306±13. The adverse effects of B. patulus on the peak abundances of B. calyciflorus were higher at low food concentration. Data on egg ratios (eggs female^–1) revealed an inverse relation with population abundance of both tested rotifer species. Our results indicated that the rate of population increase of a species was not a good indicator of its competitive ability. Instead, the ability to reproduce under continuously diminishing food resources (until a threshold level) was responsible for the competitive edge of B. patulus over B. calyciflorus. This was further influenced by the relative inoculation densities of the tested rotifer species and the offered food densities.     

The effect of prey morphology on the feeding behaviour and population growth of the predatory rotifer Asplanchna sieboldi: a case study using five species of Brachionus (Rotifera)

1. We investigated the numerical response, functional response and prey preference of Asplanchna sieboldi to five different prey brachionids. We also analysed the feeding behaviour of the predator in terms of encounters, attacks, capture and prey ingested per unit time. 2. The five prey species (Brachionus havanaensis, B. rubens, B. patulus, B. macracanthus and B. calyciflorus) differed in their body size and spine length. 3. The population growth rates of A. sieboldi ranged from 0.074 ± 0.03 to 0.431 ± 0.02 depending on prey type and density. There was a significant impact of the spine length rather than body size per se on the population growth rates of the predator. 4. The maximum number of prey consumed depended on both body size and spine length. In the functional response analyses, the plateau was reached at a prey density of 4–8 ind. mL^?1. 5. There was a significant impact of prey density on the prey preference of the predator.     

Effects of NaCl salinity on the population dynamics of freshwater zooplankton (rotifers and cladocerans)

Salinization of freshwater bodies due to anthropogenic activity is currently a very serious problem in Mexico. One of the consequences may be changes in the rotifer and cladoceran populations, both of which are generally abundant in freshwater bodies. Under laboratory conditions we evaluated the effect of different salt (sodium chloride) concentrations (0–4.5 g l⁻¹) on the population dynamics of ten freshwater zooplankton species (rotifers: Anuraeopsis fissa, Brachionus calyciflorus, B. havanaensis, B. patulus and B. rubens; cladocerans: Alona rectangula, Ceriodaphnia dubia, Daphnia pulex, Moina macrocopa and Simocephalus vetulus). All of the zooplankton species tested were adversely affected by 1.5–3.0 g l⁻¹ NaCl. In the range of salt concentrations tested, the population growth curves of B. patulus and B. rubens showed almost no lag phase and reached peak abundances within a week or two; A. fissa had a lag phase of about a week, while both B. calyciflorus and B. havanaensis started to increase in abundance immediately following the initiation of the experiments. Increased NaCl levels reduced the population abundances of A. fissa, B. calyciflorus and B. havanaensis at or beyond 1.5 g l⁻¹. NaCl at 1 g l⁻¹ had little effect on the population growth of cladocerans. M. macrocopa, which was more resistant to NaCl than the other cladoceran species, showed positive population growth even at 4.5 g l⁻¹. The rates of population increase (r, day⁻¹) were generally higher for rotifers than for cladocerans. Depending on the NaCl concentration, the r of rotifers ranged from +0.57 to −0.58 day⁻¹, while the r for cladocerans was lower (+0.34 to −0.22 day⁻¹).     

Small prey size offers immunity to predation: a case study on two species of <Emphasis Type="Italic">Asplanchna</Emphasis> and three brachionid prey (Rotifera)

We tested the hypothesis that small prey can coexist with large predators. For this we confronted two predators (smaller Asplanchna brightwellii: 900 μm and larger A. sieboldi: 1400 μm) with three prey rotifers (smaller: Anuraeopsis fissa (70 μm); larger: Brachionus calyciflorus (200 μm) and intermediate: B. patulus (120 μm)) using functional response, prey preference, population growth and life table demography. Regardless of prey type, A. sieboldi was able to consume more prey than A. brightwellii and it consumed higher number of B. patulus than of B. calyciflorus or A. fissa. Prey preference experiments showed that A. brightwellii had no preference for B. calyciflorus regardless of prey density, while A. sieboldi preferred B. calyciflorus and avoided A. fissa. Data on population growth showed that A. brightwellii was always numerically more abundant than A. sieboldi. Prey type had a significant effect on peak abundances of A. sieboldi but not of A. brightwellii. Life table demography data revealed a significantly lower lifespan in A. brightwellii fed B. calyciflorus, compared to B. patulus, but not when compared to A. fissa. A. sieboldi lifespan was not affected by prey type. Depending on prey type and predator species, generation time varied from 2 to 3 days. Both lowest (0.38 d⁻¹) and highest (0.98 d⁻¹) population growth rates were observed in A. sieboldi. We suggest that reduced reproductive output in Asplanchna was caused by either large (B. calyciflorus) or small (A. fissa) prey. At natural densities of Anuraeopsis, it is unlikely that Asplanchna reaches abundances high enough to exterminate this prey. By its extremely small size (combining low energetic profitability with low encounter rates with predators) A. fissa may coexist with Asplanchna in nature. Dedicated to H. J. Dumont for his 65th year. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont and R. Rico-Martínez Advances in Rotifer Research     

Chronic toxicity of methylparathion to the rotifer Brachionus calyciflorus fed on Nannochloris oculata and Chlorella pyrenoidosa

The effect of sublethal levels of methylparathion (0, 1, 3, 5, 7 mg l^–1) on the freshwater rotifer, Brachionus calyciflorus, during their entire life cycle was studied. Rotifers were fed on two species of unicellular algae: Nannochloris oculata and Chlorella pyrenoidosa; both algal concentrations were 5 × 10⁵ cell ml^–1.The parameters used to determine the toxicity of this compound were survival, fecundity, net reproductive rate (R)_o, generation time (T), intrinsic rate of natural increase (r), reproductive value (V _x/V_o) and life expectancy at hatching (eo). All the demographic parameters studied were affected by methyl-parathion exposure on rotifers fed on both species of algae, but the toxic effect was larger when animals were fed on Chlorella pyrenoidosa; in this case, animals showed a decreased in fertility and also a delayed first reproduction. Sublethal methylparathion levels produced a reduction in most of the parameters selected, especially after exposure to 7 mg l^–1, where the animals died before reproducing.     

Predation on Ceriodaphnia cornuta and Brachionus calyciflorus by two Mesocyclops species coexisting in Barra Bonita reservoir (SP,Brazil)

Feeding experiments with two species of carnivorous copepod, Mesocyclops longisetus (Thiebaud) and Mesocyclops kieferi Van de Velde from Barra Bonita, a eutrophic reservoir in São Paulo, Brasil, were performed using two common types of prey: Ceriodaphnia cornuta, a cladoceran, with a mean body length of 464 µm (including spines) or 393 µm (without spines), and Brachionus calyciflorus, a rotifer with a mean body length of 350 µm (including spines) or 279 µm (without spines).Both species showed higher consumption rates on Brachionus than on Ceriodaphnia. For Mesocyclops longisetus, the average rates were: 2.19 prey ind^–1 h^–1 (Brachionus), and 1.30 prey ind^–1 h^–1 (Ceriodaphnia). For Mesocyclops kieferi, the rates were 1.85 prey ind^–1 h^–1 (Brachionus) and 0.60 prey ind^–1 h^–1 (Ceriodaphnia). These experimental data are discussed with reference to the dynamics of the predator and prey populations in the reservoir.Laboratorio de Limnologia, Departamento de Ciencias Biologicas, Universidade Federal de São CarlosCentro de Recursos Hidricos e Ecologia Aplicada Lab. de Limnologia, Departamento de Hidraulica e Saneamento, Escola de Engenharia de São Carlos, Universidade de São Paulo     

Life History Characteristics of <Emphasis Type="Italic">Asplanchnopus multiceps</Emphasis> (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.     

Combined effects of food level and inoculation density on competition between Brachionus patulus (Rotifera) and the cladocerans Ceriodaphnia dubia and Moina macrocopa

Competition among cladocerans and rotifers is of considerable interest not only due to their close similarity in life history strategies, but also due to the considerable overlap they exhibit in their feeding habits. In tropical waterbodies, several genera of cladocerans, including Ceriodaphnia and Moina occur, simultaneously with rotifers. We tested over a period of 3 weeks the combined effects of food (0.5×10⁶ and 1.5×10⁶ cells ml^–1 of Chlorella) level and rotifer density on the competition between B. patulus and C. dubia and M. macrocopa using population growth experiments. For each cladoceran species we used 30 test jars of 50 ml capacity. The initial density of cladocerans was 0.2 ind ml^–1, while for B. patulus it was either 1 ind ml^–1 or 5 ind ml^–1. Neither the maximal population density nor the rate of population increase (r) of C. dubia was significantly affected by B. patulus. However, for M. macrocopa, both these variables were negatively affected by the rotifers. The combined effects of low food level and high initial density of B. patulus resulted in a 50% reduction in the peak population density of M. macrocopa. The population growth of B. patulus was negatively influenced by the presence of C. dubia and M. macrocopa. The results of the competition experiments conducted in the present study between cladocerans and rotifers suggest the existence of a more complex and delicate interaction than is generally thought.