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.     

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.     

Interrelations of rotifers with predatory and herbivorous Cladocera: a review of Russian works

Publications in Russian on the influence of epibiontic rotifers on cladocerans as well as on predator-prey interrelations between cladocerans and rotifers are reviewed. Proales daphnicola and Brachionus rubens are common epibionts of Cladocera. The biology of these species is described including the choice of host, feeding, reproduction and impact on the host. Representatives of the families Daphniidae and Moinidae are most readily colonized. At high densities of epibiontic rotifers, a high percentage of young Cladocera die. Predators consuming cladocerans (mainly Bosminidae) belong to the family Asplanchnidae. Rotifers are consumed by the predatory cladocerans Leptodora and Polyphemus. The results of functional response experiments by the author with P. pediculus feeding on Synchaeta pectinata, Asplanchna priodonta, colonial and solitary Conochilus unicornis, and Platyias patulus are given. In the range of prey densities of 100–1600 1^–1 a functional response was found in all the rotifers except P. patulus. Colonies of C. unicornis were not consumed. The highest level of feeding rate saturation was observed in Synchaeta pectinata. Various adaptations in prey morphology prevented effective predation: coloniality, large size of the prey and hard lorica with spines.     

Predatory and toxic effects of the turbellarian (Stenostomum cf leucops) on the population dynamics of Euchlanis dilatata,Plationus patulus (Rotifera) and Moina macrocopa (Cladocera)

Catenulid turbellarians, common in shallow, tropical ponds, affect their rotifer prey via the production of toxins. There is, however, no quantitative information on their effect on the demography of their prey. Here, we test the impact of Stenostomum cf leucops on the population dynamics of the rotifers Euchlanis dilatata and Plationus patulus, and the cladoceran Moina macrocopa. Experiments were initiated with rotifers at 0.5 ind. ml⁻¹ and the cladoceran at 0.2 ind. ml⁻¹; growth patterns were compared in the absence and presence of worms (2 Stenostomum ind. per 50 ml). Results revealed that brachionids were most adversely affected: there was a lower growth rate of the rotifers in the presence of worms (P < 0.01, repeated measures ANOVA), although at the densities applied, the predator did not wipe out its prey. These littoral predators may therefore regulate rotifer prey in natural conditions. In Moina, the population evolved differently; initially, we found no difference between control and treatment, but after about 10 days, the population collapsed, irrespective of a direct or indirect contact with the predator. This delayed effect deserves more study, as it could represent flatworm toxin accumulation by the cladoceran.     

Diurnal vertical distribution of rotifers (Rotifera) in the Chara zone of Budzyńskie Lake,Poland

Diurnal vertical distribution of rotifers was investigated in the Chara bed and the water immediately above it in the shallow region (ca. 1 m depth) of Budzyskie Lake (Wielkopolski National Park, Poland) in early September 1998. Eighty one rotifer species were identified – 71 among Chara and 59 in the open water. Significant differences in rotifer densities were observed in the Chara, with highest numbers during the day (2316 ind. l^–1) and lowest numbers early morning (521 ind. l^–1) and at dusk (610 ind. l^–1). Above the Chara, the numbers of rotifers did not change significantly (615–956 ind. l^–1). Littoral- or limnetic-forms differed in their diel vertical distribution between both zones. One group of littoral species was characterized by increased densities in the Chara in the daytime, while a second group increased in density during the night. The densities of limnetic species, which were much higher in open water, decreased in the morning or daytime in this zone. These differences in the diel behaviour of particular groups of rotifers may be dependent on microhabitat and may also be related to different kinds of predation, the exploitative competition for shared food resources between rotifers and crustaceans, as well as typical adaptation to littoral or limnetic life.     

Effect of Aeolosoma sp. (Aphanoneura: Aeolosomatidae) on the Population Dynamics of Selected Cladoceran Species

Although oligochaete worms naturally coexist with cladocerans in many shallow freshwater ponds and lakes, their influence on the latter is not well established. In this work we studied the effect of Aeolosoma sp. on the population growth of Alona rectangula, Ceriodaphnia dubia, Daphnia pulex, Macrothrix triserialis and Moina macrocopa. Population growth studies were conducted at one algal food density (1 × 10⁶cells ml^–1 of Chlorella vulgaris). The experimental design was similar for all five cladoceran species, where we used 100 ml capacity transparent jars containing 50 ml of EPA medium with the desired algal density and three replicates for each treatment. The test medium was changed daily and fresh algal food was added. The initial density of each of the cladoceran species in the population growth studies was 0.4 ind ml^–1 while that of the worms 1.0 ind ml^–1. Following inoculation, we estimated daily the number of cladocerans and the worms for duration of 21 days. Regardless of the presence of worms, Moina macrocopa and Macrothrix triserialis showed rapid population growth while A. rectangula took more than 2 weeks to reach peak abundances. With the exception of M. triserialis, all the other our cladoceran species declined in the presence of Aeolosoma sp. The lowest peak population density (about 1 ind ml^–1) was observed for M. triserialisin controls. The remaining species had peak densities of about 3–5 ind ml^–1. The rates of population increase per day varied from 0.03 to 0.19 depending on the cladoceran taxa and the treatment. In general we found that pelagic taxa were more adversely affected by the presence of the worms than were the littoral cladocerans.     

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).     

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.     

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.     

Screening Methods for Improving Rotifer Culture Quality

We studied whether the selection of rotifer B. plicatilis strain (Japanese, Russian or Australian), as well as the addition of gamma-aminobutyric acid (GABA) to the culture water, are useful in stabilising rotifer cultures. We examined the effect of a combination of the following stressors: unionized ammonia (2.4 mg l⁻¹), contamination by protozoa Euplotes sp. (10 cells ml⁻¹) and addition of methyl cellulose to increase the culture water viscosity at 15 cp. Rotifer reproductive tests and enzyme activity measurements (glucosidase) were conducted to determine the effect of the treatments. All tests were conducted at 25 °C and rotifers were fed Nannochloropsis oculata at 7 × 10⁶ cells ml⁻¹. The combined effects of the stressors caused a significant decrease in lifespan, fecundity and glucosidase activity. The effect of the stressors on reproductive characteristics and glucosidase activity could be neutralized if rotifers were treated with GABA.     

A laboratory study on the demography and competition of three species of littoral cladocerans from Lake Huetzalin,Xochimilco, Mexico

Lake Huetzalin (Xochimilco) is a shallow waterbody in Mexico City with a dominance of the littoral cladocerans Simocephalus vetulus, Alona rectangula, and Pleuroxus aduncus. Here, we present data on the life table and population dynamics of these three cladoceran species on two types of diets (alga Scenedesmus acutus and detritus). We also analyzed the effect of competition on population growth using Scenedesmus, which proved to be the better diet than detritus. We hypothesize that feces of the larger cladoceran species (S. vetulus) could be a food source for the smaller taxa (A. rectangula and P. aduncus). Experiments were conducted in 100 ml beakers with 50 ml of medium in each at a temperature of 23 ± 1°C. We studied the demography of three test species using two food types with four replicates for each treatment in the life table experiments. Based on the data thus obtained we further studied the competition between the cladocerans on a diet of Scenedesmus. The maximal growth rates obtained for P. aduncus, S. vetulus, and A. rectangula fed Scenedesmus acutus were: 0.1, 0.3, and 0.5 d⁻¹, respectively. Among the test species, S. vetulus had the highest survivorship of 77 days when cultured on detritus and P. aduncus the lowest of 20 days on Scenedesmus. The gross reproductive rate (offspring female⁻¹ lifespan⁻¹) ranged from 6 neonates in Pleuroxus to 160 in Simocephalus per female. From the competition experiments conducted using alga as the diet, we observed that A. rectangula and, especially P. aduncus, had higher growth rates in the presence of S. vetulus than in the controls. This facilitation could be one of the reasons for higher species diversity in the littoral of water bodies.     

Experimental study on the impact of dinoflagellate Alexandrium species on populations of the rotifer Brachionus plicatilis

To investigate harmful effects of the dinoflagellate Alexandrium species on microzooplankton, the rotifer Brachionus plicatilis was chosen as an assay species, and tested with 10 strains of Alexandrium including one known non-PSP-producer (Alexandrium tamarense, AT-6). HPLC analysis confirmed the PSP-content of the various strains: Alexandrium lusitanicum, Alexandrium minutum and Alexandrium tamarense (ATHK, AT5-1, AT5-3, ATCI02, ATCI03) used in the experiment were PSP-producers. No PSP toxins were detected in the strains Alexandrium sp1, Alexandrium sp2.Exposing rotifer populations to the densities of 2000 cells ml⁻¹ of each of these 10 Alexandrium strains revealed that the (non-PSP) A. tamarense (AT-6) and two other PSP-producing algae: A. lusitanicum, A. minutum, did not appear to adversely impact rotifer populations. Rotifers exposed to these three strains were able to maintain their population numbers, and in some cases, increase them. Although some increases in rotifer population growth following exposures to these three algal species were noted, the rate was less than for the non-exposed control rotifer groups.In contrast, the remaining seven algal strains (A. tamarense ATHK, AT5-1, AT5-3, ATCI02, ATCI03; also Alexandrium sp1 and Alexandrium sp2) all have adverse effects on the rotifers. Dosing rotifers with respective algal cell densities of 2000 cells ml⁻¹ each, for Alexandrium sp1, Alexandrium sp2, and A. tamarense strains ATHK and ATCI03 showed mean lethal time (LT₅₀) on rotifer populations of 21, 28, 29, and 36h, respectively. The remaining three species (A. tamarense strains AT5-1, AT5-3, ATCI02) caused respective mean rotifer LT₅₀s of 56, 56, and 71 h, compared to 160 h for the unexposed “starved control” rotifers. Experiments to determine ingestion rates for the rotifers, based on changes in their Chlorophyll a content, showed that the rotifers could feed on A. lusitanicum, A. minutum and A. tamarense strain AT-6, but could graze to little or no extent upon algal cells of the other seven strains. The effects on rotifers exposed to different cell densities, fractions, and growth phases of A. tamarense algal culture were respectively compared. It was found that only the whole algal cells had lethal effects, with strongest impact being shown by the early exponential growth phase of A. tamarense. The results indicate that some toxic mechanism(s), other than PSP and present in whole algal cells, might be responsible for the adverse effects on the exposed rotifers.     

Comparative population growth and life table demography of the rotifer Asplanchna girodi at different prey (Brachionus calyciflorus and Brachionus havanaensis) (Rotifera) densities

Population growth and life table demography of the predatory rotifer A. girodi using spineless Brachionus calyciflorus and spined Brachionus havanaensis as prey at densities of 1, 2, 4 and 8 ind. ml^–1 at 25°C were studied. Regardless of the prey species, the population of A. girodi increased with increasing availability of Brachionus in the medium. At any given prey density, A. girodi fed B. calyciflorus showed consistently better growth than when fed B. havanaensis. The maximum population densities of A. girodi varied from 0.28 to 1.8 ind. ml^–1 depending on the prey species and the density. The rate of population increase observed in population growth studies varied from 0.17 to 0.43 day^–1 when fed B. calyciflorus and 0.09 to 0.27 day^–1 when fed B. havanaensis. Male population of A. girodi was closely related to female density. The lowest average lifespan was observed for A. girodi when fed B. havanaensis at 1 ind. ml^–1, while the converse was the case when fed B. calyciflorus at comparable prey concentration. Net reproductive rates varied from 16 to 26 offspring female^–1 lifespan^–1 depending on the prey species and concentration. Generation time of A. girodi decreased with increasing food concentrations for both the prey species. The rates of population increase obtained from life table demography were lower for A. girodi when fed B. havanaensis than when fed B. calyciflorus.     

Interaction Among Copper Toxicity,Temperature and Salinity on the Population Dynamics of <Emphasis Type="Italic">Brachionus Rotundiformis</Emphasis> (Rotifera)

Heavy metals may interact with ecological factors such as temperature, food level and salinity, causing both mortality and reduced reproduction in organisms. Among different heavy metals, copper compounds are commonly used for eliminating algal blooms in aquaculture tanks. At certain concentrations, copper is toxic to rotifers. In the present work, we evaluated the combined effects of salt concentrations (2.5 and 5.0 g l⁻¹ NaCl), copper levels (0, 0.03125, 0.0625, 0.125 and 0.25 mg l⁻¹ as CuCl₂) and two temperatures (20 and 25 °C) on the population growth of B. rotundiformis using Chlorella as the algal food (at 0.5 × 10⁶ cells ml⁻¹ for every 24 h). Regardless of salinity and temperature, copper at concentrations as low as 0.03 mg l⁻¹ had an adverse effect on the population growth of rotifers and above 0.125 mg l⁻¹, the populations did not grow. The effect of the toxicant on B. rotundiformis was more severe at 25° than at 20 °C at lower salinity. In general, we observed peak densities of rotifers around day 12 at 20 °C but 6–8 days earlier at 25 °C. Peak population densities of B. rotundiformis in the controls at the salinity of 2.5 g l⁻¹ ranged from 90 to 180 ind. ml⁻¹, depending on temperature; at a salinity of 5.0 g l⁻¹, these were lower. The population growth rates, r, in our study varied from +0.31 to –0.12 depending on the test conditions. There was a significant impact of temperature, salinity and toxicity level on the population growth rate of B. rotundiformis. Our results suggested that even narrow changes in salinity could negatively influence the toxicity of heavy metal on the population growth rates of B. rotundiformis.     

Observations on feed size and capture success in the larval butterfly splitfin (Ameca splendens Miller & Fitzsimons, 1971, Pisces: Goodeidae) reared on zooplankton

In this study, we quantified the feeding behaviour (encounter, attack, capture. and ingestion) of larval A. splendens on micro‐crustacean prey [cladocerans: Alona rectangula, Simocephalus vetulus (separately neonates and adults), Ceriodaphnia dubia, Daphnia pulex (juveniles), Moina macrocopa and ostracods: Heterocypris incongruens]. Although we initially (first 4 weeks) offered rotifers (Brachionus calyciflorus and B. patulus), they were not consumed by the larvae and hence observations with these prey were discontinued. Feeding behaviour was observed during the first 10 weeks. Fifteen observations were made with each prey species (seven diets × four replicates). Experiments were conducted in 50 ml transparent containers with 20 ml fish‐conditioned water into which one fry was introduced. Before introducing the fish, 20 individuals of a given cladoceran prey species or 50 individuals of a rotifer prey species were introduced. Until the fourth week, we used 20 ml of medium and thereafter 30 ml, but the prey density used remained constant (1 ind. ml⁻¹). Observations (10 min per fry per cladoceran replicate) were taken under a stereomicroscope (20×) for the first 2 weeks and later with a lamp and a magnifying lens. The number of encounters (E), attacks (A), captures (C) and ingestions (I) were recorded. During the study period, there was a 60% increase in gape size but only a 30% increase in body length. The number of encounters of larval A. splendens was highest (192) on M. macrocopa and lowest (29) on ostracods and adult S. vetulus (59). The inverse relationship between capture success and prey size was more pronounced during the latter half of the study period. Compared with all the other prey types offered, A. splendens fed maximally on M. macrocopa, which therefore could be a suitable diet for the larval rearing of this fish species.     

Patterns of prey selectivity in the cyclopoid copepod Mesocyclops thermocyclopoides

In the shallow and eutrophic subtropical aquatic ecosystems, which it generally inhabits, the omnivorous copepod Mesocyclops thermocyclopoides encounters a wide variety of animal prey types including ciliates, rotifers, and cladocerans. We studied prey selectivity in laboratory-reared adult females of this species given a choice of (i) prey types belonging to different taxa (ciliates, rotifers, cladocerans, and cyclopoid nauplii), and (ii) different prey species within a taxonomic group differing in body size, morphology or behaviour. We also tested the effect of different proportions of prey species on its selectivity. Prey type proportion had no significant effect on selectivity of the copepod, nor was there any evidence of switching based on the relative abundance of prey. Among the ciliate prey species tested, the largest species, Stylonychia mytilus was positively selected regardless of its relative abundance, while the smallest, S. notophora was selected only when its density was higher. Offered a choice of three species of a brachionid rotifer differing in size, the copepod selected the largest of them, Brachionus calyciflorus, and avoided the smallest B. angularis. The evasive rotifer Hexarthra mira was also avoided. When prey choice included three cladoceran species Daphnia similoides, Moina macrocopa and Ceriodaphnia cornuta, the copepod selected the intermediate-sized M. macrocopa regardless of the abundance of the other two species. Although it fed on Mesocyclops nauplii when there was no choice, M. thermocyclopoides avoided them when alternative food was available. In a multispecies prey choice test, the copepod selected predominantly the rotifer B. calyciflorus and the cladoceran M. macrocopa. We suggest that the prey selectivity patterns shown by M. thermocyclopoides are adaptive in that they lead to ingestion of the most profitable prey.     

Prey selection by the larvae of three species of littoral fishes on natural zooplankton assemblages

The prey selection of larvae of three common littoral fish species (pike, Esox lucius; roach,Rutilus rutilus; and three-spined stickleback, Gasterosteus aculeatus) was studied experimentally in the laboratory by using natural zooplankton assemblages. Zooplankton prey was offered at four different concentrations to study the functional responses of the planktivores. The diets of pike and sticklebacks were formed mainly of copepod juveniles and adults, which dominated the prey communities, although sticklebacks ate also cladocerans. The diet of roach larvae consisted of rotifers, cladocerans and copepods, without prey selection, in equal proportions indicating a more omnivorous diet. All fish larvae were able to feed selectively although in sticklebacks prey selection was less pronounced. Pike and roach larvae preferred large prey to smaller prey types. Patterns of prey selection are discussed in the context of size-selection theory and apparent vs. true selection.     

Feeding the fairy shrimp Streptocephalus (Anostraca - Crustacea) with the rotifer Anuraeopsis

Laboratory-cultured Streptocephalus torvicornis were offered 8 concentrations (from 6 to 800 ind. ml^–1) of Anuraeopsis fissa for periods of 2 h 30 min. Two size classes, small (male: 14.7 mm± 1.6, female: 15.4 mm± 1.3) and large (male: 20.0 mm±2.0, female: 23.1 mm± 1.5), of S. torvicornis were used. Functional response for large S. torvicornis (both sexes) plateaued at 400 rotifers ml^–1, while in small specimens it did so at 200 prey ml^–1. Females consumed significantly more (30%) prey than males. Large males consumed maximum 4730 rotifers h^–1, females 6560 h^–1.     

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.     

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